PHASE TRANSITIONS IN OVERSATURATED SOLID SOLUTION OF NITROGEN IN THE α-ZR OBTAINED BY USING HYDROGEN TERMOEMISSION Текст научной статьи по специальности «Химические науки»

ВОДОРОДНАЯ ЭКОНОМИКА

rJ

HYDROGEN ECONOMY

КОНСТРУКЦИОННЫЕ МАТЕРИАЛЫ

STRUCTURAL MATERIALS

PACS: 61.05.fm; 61.66.Fn

ФАЗОВЫЕ ПРЕВРАЩЕНИЯ В ПЕРЕСЫЩЕННОМ ТВЕРДОМ РАСТВОРЕ АЗОТА В a-Zr, ПОЛУЧЕННОМ ПУТЕМ ТЕРМОЭМИССИИ ВОДОРОДА

12 12 И. Хидиров , Т.Н. Везироглу , Н.Н. Мухтарова , А. Везироглу

1Институт ядерной физики НАН Узбекистана 100214, Ташкент, Узбекистан, e-mail: khidirov@inp.uz Исследовательский институт чистой энергии, Университет Майами 33124, США, Флорида, Корал Гейблс, e-mail: veziroglu@miami.edu

Методом дифракции нейтронов (к = 0,1085 нм) на примере твердого раствора азота и водорода в решетке a-Zr (ZrN0-43H0-38) показано, что синтез водородсодержащих фаз внедрения с последующим низкотемпературным удалением водорода в непрерывно откачиваемом высоком вакууме при температурах Тэв < Траспада позволяет инициировать ряд фазовых превращений, которые невозможно осуществить традиционными методами (длительным отжигом или закалкой). Таким способом получены три модификации пересыщенного твердого раствора ZrN0,43, не существующие на равновесной фазовой диаграмме. Между этими модификациями обнаружены энантиотропные и монотропные фазовые переходы. Изучены кристаллические структуры обнаруженных фаз. Предложена схема обратимых и необратимых фазовых превращений на основе индуцированной водородом фазы.

В индуцированной водородом фазе наблюдается изотропное сжатие гексагональной кристаллической решетки матрицы по сравнению с изоструктурной водородсодержащей фазой. Обнаружено, что смещение атомов металла относительно идеального положения в водородсодержащей и индуцированной водородом фазах происходит в разных направлениях. В водородсодержа-щей фазе атомы металла смещаются к плоскости, заполненной атомами азота, а в индуцированной водородом фазе - к плоскости, содержащей азотные вакансии. Это объясняется различным характером химической связи в водородсодержащем соединении и соединении без водорода.

PHASE TRANSITIONS IN OVERSATURATED SOLID SOLUTION OF NITROGEN IN THE a-Zr OBTAINED BY USING HYDROGEN TERMOEMISSION

I. Khidirov1, T.N. Veziroglu2, N.N. Mukhtarova1, A. Veziroglu2

'Institute of Nuclear Physics, Uzbekistan Academy of Sciences, 100214, Tashkent, Uzbekistan, e-mail: khidirov@inp.uz 2Clean Energy Research Institute, University of Miami, Coral Gables, Florida, 33124, USA, e-mail: veziroglu@miami.edu

By neutron diffraction (X = 0.1085 nm) it has been demonstrated with an example of the solid solution of nitrogen and hydrogen in a lattice of a-Zr (ZrN043H0 38) that the synthesis of hydrogenous interstitial phases, followed by low-temperature removal of hydrogen under continuous evacuating, makes possible to obtain metastable interstitial phases which cannot be obtained by traditional methods (long annealing or hardening). In this way three modifications of oversaturated solid solution ZrN043 which are absent in the equilibrium phase diagram has been obtained. Between these phases enantiotropic and monotropic phase transitions were found. The scheme of reversible and nonreversible phase transformations is offered on a basis of the hydrogen induced phase.

In the hydrogen induced phase the isotropic compression of HCP crystal lattice in comparison with the isostructural hydrogenous phase is observed. It is found that the displacement of metal atoms from their ideal position occurs in different directions in the hydrogenous and in the hydrogen-induced phases. In the hydrogenous phase the metal atoms shift toward a plane filled with nitrogen atoms, and in the hydrogen induced phase - toward a plane containing nitric vacancies. This may be explained by various characters of chemical bonding forces in the hydrogenous compound and in the hydrogen-free one.

Information about the author: Head of the laboratory of composite materials of the Institute of nuclear physics of Uzbekistan Academy of Sciences.

Education: Tashkent State Pedagogical University (1969 r.), doctor of sciences in physics and mathematics (1998), professor (2007).

Main range of scientific interest: structural phase transformations, interstitial alloys and solid solutions (carbides, nitrides, hydrides), intermetals and hydrides of intermetals, superconducting ceramics and other ceramics.

Publications: more 120 articles, 4 patents.

Irisali Khidirov

International Scientific Journal for Alternative Energy and Ecology № 2 (70) 2009

© Scientific Technical Centre «TATA», 2009

T. Nejat Veziroglu

Information about the author: President of International Association for Hydrogen Energy Association, Miami, USA; Director of Clean Energy Research Institute of University of Miami, USA. Education: A.C.G.I., Mechanical Engineering, 1946. B.Sc. Mechanical Engineering, 1946. D.I.C., Advanced Studies, London, 1947. Ph.D. - Heat Transfer, 1951. Professor, 1962.

Main range of scientific interest: hydrogen energy, interstitial heat transfer, environment, hydrogen storage and environmental impact of energy.

Publications: number of papers in refereed journals - 224; number of communications to scientific meetings - 155; number of books: 2 books, 84 proceedings.

Information about the author: Senior researcher of the laboratory of composite materials of the Institute of nuclear physics of Uzbekistan Academy of Sciences.

Education: Nizhni Novgorod State University (1958); PhD, 1982; the specialist in the field of X-ray structure analysis and crystal chemistry.

Main range of scientific interest: investigations into the effect of ionizing radiation (y-rays, neutrons) on the structure and properties of materials (solid solutions, HTS-ceramics, semiconductors, etc.). Publications: more 70 articles.

Nina Nikolaevna Mukhtarova

Information about the author: PhD student, The Instituto Superior Tecnico, Lisbon, Portugal. Education: University of Miami, Management of Technology, M.S., 2000-2004. University of Miami, Intensive English Program, 1999-2000. Marmara University, Marketing and International Enterprises, B.S., 1992-1999. Main range of scientific interest: hydrogen production from H2S; hydrogen transportation; hydrogen technology transfer. Publications: 10 articles.

Ayfer Veziroglu

Introduction

It is known that in the Zr-N-H system the ordered phase Zr2N1-xHy with structure of anti-CdI2 type (on nitrogen) is formed [1, 2]. Hydrogen has both small nuclear weight and binding energy and also high diffusion rate. These factors allow one to obtain oversaturated solid solution (sol. s.) ZrNx by evacuation of hydrogen out of three-component sol. s. ZrNxHy at rather low temperatures [3]. Dehydrogenation of the sol. s. under persistent evacuation at temperature 400° C results in full removal of hydrogen out of a lattice with conservation of the structure type [4]. However, with that, formation of the second, orthorhombic ordered phase with structure of anti-CaCl2 type (space group -sp. gr. Pnnm) on a basis of oversaturated sol. s. ZrNx is observed. It is of interest the vacuum evacuation of hydrogen out of a lattice of the ordered sol. s. ZrN0 43H038 (sp. gr. P3m1) at temperature lower than temperatures of orthorhombic ordering or formation of disordered hexagonal sol. s.: Tev <Tortho < Tdisord. It may be supposed that because of low temperature of hydrogen evacuation, in the Zr-N system the ordered

induced by hydrogen structure of sol. s. ZrNx (sp. gr. P3m1) will be "frozen" which is not observed under usual conditions. The analogous way of obtaining the phase induced by hydrogen in the Ti-N system has been offered in [5]. Besides, it is of interest to study mutual transformations in hydrogen-induced metastable and stable phases of the Zr-N system.

Thereby the aim of the present work was neutron diffraction investigation of hydrogen-induced phases (HIPh) and mutual transformations of metastable and stable phases in an example of the Zr-N system.

Experiment techniques

Neutron diffraction experiment was carried out using the neutron diffractometer mounted at a thermal column of atomic reactor WWR-SM of the Institute of Nuclear Physics of Uzbekistan AS (X = 0.1085 nm) [6]. Calculation of structural characteristics was carried out by Rietveld full-profile method [7, 8] using neutron diffraction data. X-ray diffraction patterns were obtained using the X-ray diffractometer (X = 0.15418 nm).

The sol. s. ZrN0 43H038 for studying was prepared by self-propagating high-temperature synthesis (SHS) [9]. SHS of inorganic compounds is based on exothermal reaction of initial reagents. As initial materials nitrogen of the "extra-pure" brand and Zr powder of the M-41 brand were taken. According to nameplate, the Zr powder contained 0.45 mas % of hydrogen. The pressure of nitrogen in a constant pressure bomb was 700 atm. After synthesis the sample was exposed to homogenizing annealing in evacuated and sealed-off quartz ampoule at temperature 1000° C during 12 h. Then for fixation of the high-temperature state samples were hardened in air. Powder samples with grain sizes no more than 60 ^m were studied.

Samples composition was determined by chemical analysis and was controlled by minimizing the R factors of structure determination using neutron diffraction patterns. Dehydrogenation of hydrogenous compounds was carried out under persistent evacuation (at vacuum not more than 5.3-10-3 Pa). After dehydrogenating at every temperature a neutron diffraction pattern was surveyed. Hydrogen quantity in samples was watched as decline of incoherent background caused by incoherent neutron scattering on H nuclei. Since hydrogen nucleus has the largest amplitude of incoherent neutron scattering, its background scattering falls away with the increase of Bragg angle [10]. The hydrogen content was also estimated by the analysis of reflection intensities and sometimes by chemical analysis.

Experiment results

As an initial sample the single-phase ordered solid solution ZrN0.43H0,38 was taken. According to X-ray structure analysis, the sample was single-phase and homogeneous and had hexagonal close-packed structure (HCP) with unit cell parameters: a = 0.3274 ± 0.0002; c = 0.5321 ± 0.0003 nm; c/a =1.625.

The neutron diffraction pattern of the initial sol. s. ZrN0,43H0,38 is shown in Fig. 1, a. Uniform slope of incoherent background points to hydrogen presence. The neutron diffraction data of the sol. s. show that nitrogen atoms are ordered over one of two types of octahedral

interstices alternating along c axis, and hydrogen atoms - over tetrahedral interstices of hexagonal close-packed (HCP) metal structure; the unit cell parameters are: a = 0.3274; c = 0.5321 nm (c/a = 1.625). The metal atoms (zZr = 0.243) are displaced along c axis from their ideal positions (zid = 1/4) toward the plane of 1 (a) octahedrons occupied with nitrogen atoms (Table 1). So, the initial sample is the ordered sol. s. of nitrogen and hydrogen in HCP structure of a-Zr: it is a'-Zr2N086H076 phase, sp. gr. P3m1, structure of anti-CdI2 type on nitrogen.

Рис. 1. Нейтронограммы твердых растворов: a - Zr2N0,86H0,76 (а'; пр. гр. Р3m1 ); b - Zr2No,86 (a'; пр. гр. P3m1 ); c - Zr2No,86 (a''; пр. гр. Pnnm); d - ZrNo,43 (L3' ; пр. гр. Р6з/ттс);

e - смесь a- и 8-фаз Fig. 1. Neutron diffraction patterns of sol. s.: a - Zr2N0.86H0.76 (a'; sp. gr. P3m1 ); b - Zr2No.86 (a'; sp. gr. P3m1 ); c - Zr2N0 ,86 (a''; sp. gr. Pnnm); d - ZrNo,43 ( L3' ; sp. gr. Р6з/ттс); e - mixture of a and 8 phases

Таблица 1

Структурные характеристики и факторы недостоверности R исходного твердого раствора

ZrN0,43H0,38 в рамках пр. гр. P3m1

Table 1

Structure characteristics and discrepancy indices R of the initial sol. s. ZrN0 43H0 38 in the sp. gr. P 3m1

Atom Position Atom coordinates B, nm2 AB, nm2 n An

x y z Az

Zr 2 (d) 1/3 2/3 0.243 0.001 0.0039 0.0005 2

N 1 (a) 0 0 0 0.0072 0.0009 0.86 0.05

H 2 (d) 1/3 2/3 0.619 0.002 0.0107 0.0042 0.76 0.04

Rp = 1.9; Rwp = 2.5; RBl = 4.4%

Notice: B - individual thermal factor; n - occupancy of a position.

the Zr-N system [11]. It should be noted that the annealing of hydrogenous Zr2No.86Ho.76 phase under similar conditions does not lead to decay or change of crystal structure. Further annealing of the disintegrated sample (mixture of a+5 phases) at 375° C for 36 h did not lead to recovery of the phase with anti-CdI2 structure. As the crystal structure of a'-Zr2No,86-phase has been stabilized by hydrogen (then all hydrogen atoms are removed) and it is not observed in the phase diagram, this phase is offered to name the hydrogen induced phase (HIPh), unlike hydrogenous one.

For HIPh a'-Zr2N0.86 the least value of factor RBr = = 6.4% is obtained with nitrogen atoms arranged over 1(a) octahedral positions (sp. gr. P3m1) with the free metal parameter zzr = 0.255 differing from the ideal value zid = 1/4 and from z = 0.243 for corresponding hydrogenous phase (Table 2). Hence, in the HIPh metal atoms displacement directions change*: from the plane filled with nitrogen atoms toward the plane of nitrogen vacancies (Fig. 2, a and b). HIPh has the following lattice parameters: a = 0.3264; c = 0.5299 nm (c/a = 1.623). These values are less than those of corresponding hydrogenous phases. It should be noted that within the limits of experiment errors the axes relation c/a is the same for HIPh and the hydrogenous phase. As a result of hydrogen removal out of hexagonal lattice its near isotropic compression occurs.

Таблица 2

Структурные характеристики и факторы недостоверности R водородом индуцированной а'-Zr2N0.86-фазы (полученной при температуре 375° C) в рамках пр. гр. P3m1

Table 2

Structure characteristics and discrepancy indices R of HIPh а'-Zr2N0.86 (obtained at temperature 375° C)

in the sp. gr. P 3m1

Dehydrogenation of the a'-Zr2Na86Ho.76 phase (of composition ZrN0.43H0.38) was carried out by a regime of step annealing within the temperature range 100° C < T < 375° C with steps of 25° C and exposure time of 24-48 h at each temperature. Vacuum in the enclosed volume the vacuum was kept below 5.3-10-3 Pa under permanent evacuation. After each vacuum annealing step, neutron diffraction pattern was obtained and quantity of hydrogen in a sample was estimated. Analysis of the neutron patterns shows that such dehydrogenation does not result in marked change of hydrogen content. Hydrogen content is confirmed by conservation of the slope of incoherent background in diffraction patterns. Though according to neutron structure analysis and vacuum extraction data dehydrogenation at T > 375° C during 36 h leads to practically complete hydrogen removal out of the sample (Fig. 1, b) while the quantity of nitrogen is kept still. The structural analysis of the dehydrogenated sample showed that the ordered phase structure of anti-CdI2 type (a'-Zr2N086) did not change.

The phase was annealed at 1000° C for 3 h in evacuated and sealed-off quartz ampoule. As a result of annealing the dehydrogenated sample disintegrated into disordered sol. s. of nitrogen in a-Zr - hexagonal aphase (structure of L' type) and a cubic 5-phase (Fig. 1, e) corresponding to the equilibrium phase diagram of

Atom Position Atom coordinates B, nm2 AB, nm2 n An

x y z Az

Zr 2 (d) 1/3 2/3 0.254 0.002 0.0043 0.0008 2

N 1 (a) 0 0 0 0.0032 0.0012 0,86 0,01

Rp = 2.9; Rwp = 3.9; RBr = 6.4%

*A relative displacement is meant, as in the coordinate origin (center of symmetry) there is a lighter nitrogen atom.

Рис. 2. Элементарные ячейки водородсодержащего твердого раствора ZrNo,86Ho,76 и фаз, инициированных с помощью низкотемпературного вакуумного извлечения водорода: 1 - атомы металла; 2 - атомы азота; 3 - азотные вакансии; 4 - атомы водорода; 5 - водородные вакансии; 6 - статистически распределенные атомы азота Fig. 2. Unit cells of the hydrogenous solid solution ZrN0.86H0.76 and of phases initiated by low-temperature vacuum extraction of hydrogen: 1 - metal atoms; 2 - nitrogen atoms; 3 - nitrogen vacancies; 4 - hydrogen atoms; 5 - hydrogen vacancies;

6 - statistic arrangement of nitrogen atoms

Analysis of neutron diffraction pattern of the phase Zr2N0.86H0.76 dehydrogenated at 450-650° C (Fig. 1, c) shows that at these temperatures the ordered orthorhombic Zr2N086 phase with structure of anti-CaCl2 type (a''-phase) is formed. The diffraction pattern of this phase has been indexed in orthorhombic system (sp. gr. Pnnm) with the lattice parameters: a = 0.5632 nm « « V ao, b = 0.5252 nm « co, c = 0.3256 nm « ao where ao and co are lattice parameters of the initial hexagonal

phase. The unit cell of this phase is shown in Fig. 2, c. A good agreement between experimental and calculated intensities (R = 6.3%) is achieved given that four Zr atoms are in positions 4 (g), the nitrogen atoms occupy mainly positions 2 (a) and partially - positions 2 (c) (Table 3). At 650° C the partial disordering of nitrogen atoms (~20%), followed by their transition to octahedral interstices of another type - 1 (c), takes place.

Структурные характеристики и факторы недостоверности R а"-Zr2N0.86-фазы (полученной при температуре 450° C) в рамках пр. гр. Р nnm

Structure characteristics and discrepancy indices R of a"-Zr2N0.86 phase (obtained at temperature 450° C) in the sp. gr. Р nnm

Таблица 3

Table 3

Atom Position Atom coordinates B, nm2 AB, nm2 n An

x Ax y Ay z

Zr 4 (g) 0.324 0.002 0.271 0.001 0 0.39 0.06 4

N 2 (a) 0 0 0 0.53 0.13 1.38 0.02

N 2 (c) 0 1/2 0 0. 53 0.13 0.34 0.02

Rp = 4.5; Rwp = 6.0; RBr = 6.7%

In neutron diagram of the sample, dehydrogenated at 800° C, has been reflections from hexagonal phase with structure of L3' -type (a-phase) (Fig. 1, d). The unit cell of the a-phase is shown in Fig. 2, d. We also investigated mutual transformation of the metastable phases. The annealing of the a'-HIPh at temperatures 450-650° C results in formation of a -phase. In other words, in this temperature range phase transition from a' to a" is observed. The annealing of the a -phase at 400-375° C during 36 h does not lead to restoration of a'-phase. Hence, a'—450°c > a'' transition is monotropic

the oversaturated sol. s. ZrN0.43 which do not exist in the equilibrium phase diagram have been obtained. In Fig. 3 these modifications are outlined by dashed lines (the small rectangle).

(nonreversible) and a'-phase can be induced only by hydrogen thermoemission at temperature below that the temperature of a' '-phase formation.

By increasing the temperature up to 750-800° C, the a' '-phase passes into a-phase (disordered oversaturated sol. s. of nitrogen in a lattice of a-Zr) with structure of L' -type (Fig. 1, d). This transition is enantiotropic (reversible) because the a -phase is formed again by annealing of the a-ZrN043 phase at temperatures 650450° C. The annealing of the phases at 1000° C results in their disintegration into to a- and 5-phases (Fig. 1, e). No any annealing of samples that have been decayed into a-and 5-phases does not lead to formation of a'-, a''- and a-phases in a single-phase kind. Therefore, the found phase transitions a' ^ a'' ^ a occur in metastable state. The formed metastable phases and phase transitions between them are schematically shown in Fig. 3.

Thus, by means of dehydrogenation of the preliminary hydrogenated sample, three modifications of

Рис. 3. Схема получения и фазовых превращений метастабильных пересыщенных твердых растворов

в системе Zr-N Fig. 3. Scheme of obtaining and phase transformations of metastable oversaturated sol. s. in the Zr-N system

International Scientific Journal for Alternative Energy and Ecology № 2 (70) 2009

© Scientific Technical Centre «TATA», 2009

Hydrogénation of sol. s. of the Zr-N system [12] reveals that under a hydrogen pressure of P>105 Pa, all samples, including biphase ones, form the sol. s. a'-Zr2N0.86H0.76 (sp. gr. P 3m1). This inverse process is also shown in Fig. 3. In these cases, the exposure time required for hydrogen saturation up to the concentration H/Zr = 0.38 will be different at various temperatures.

The scheme of reversible and nonreversible phase transformations in metastable state of ZrN043 is shown in

Fig. 4.

Рис. 4. Схема фазовых превращений на основе индуцированной водородом а'-7г21\1о,8б-фазы Fig. 4. Scheme of phase transformations on the basis of HIPh a'-Zr2No.86

Discussion of the results

The formation of observed HIPh and phase transitions in metastable state of oversaturated sol. s. of the Zr-N system may be explained as follows. Under 375° C hydrogen atoms are removed out of the lattice but the configuration of relatively heavy atoms of the matrix does not change. As a result it is possible to obtain the "frozen" structure stabilized previously by hydrogen using thermoemission. At temperatures 450° C < T < 650° C the diffusive mobility of nitrogen atoms increases, making possible the ordering type change. Further temperature rise causes large displacements of atoms from lattice sites, leads to disorder in distribution of nitrogen atoms. The disordered state occurs typically for oversaturated sol. s. This state precedes decay process starting above 800° C. At temperatures below 800° C decay process is strongly hindered, as it requires: nitrogen atoms to move at macroscopic distances and the power barrier to be overcome. It is necessary for formation of nucleus of new phase, namely, cubic zirconium nitride. Though, ordering process needs only redistribution of nitrogen atoms and their movement at distances about interatomic space. As a result, during low-temperature annealing (400° C < T < 800° C ) the oversaturated sol. s. does not decay (that would correspond to a minimum of free energy) instead passes into another ordered or disordered state corresponding to a local minimum. Hence, in sol. s. of nitrogen in a-Zr, having high decay temperatures, a number of phase transformations in metastable state can be observed at temperatures below Tdecay « 1000° C. Fig. 3 shows that depending on the temperature of hydrogen evacuation it is possible to obtain several different phases. The decay temperature for sol. s. ZrNx is higher

than for TiNx. Therefore in the Zr-N system at T < Tdecay it is possible to change the ordering type (anti-CdI2 ^ anti-CaCl2). Hence, in sol. s. of nitrogen in a-Zr, having high decay temperatures, a number of phase transformations in metastable state can be observed at temperatures below Tdecay « 1000° C (Fig. 3).

During formation of HIPh two observations are made: isotropic compression of the crystal lattice and change for metal atoms displacement directions. Apparently, such effects may be explained by various characters of chemical bond of nitrogen and hydrogen with metal atoms. It should be noted that the isotropic compression takes place in the induced phases where the structure type of initial hydrogenous phase is kept. For sol. s. TiNx [13], TiCx, TiOx [14] and ZrNx [15] it was shown that at implantation of N, C and O into hexagonal metal lattice the unit cell parameter c changes much more than a. Isotropic compression of lattice and anisotropic change of lattice parameters at implantation of N, C and O atoms in the same structure of the HIPh phase made possible to suppose that hydrogen atoms create isotropic field of elastic stresses in a lattice. It is difficult to explain the different direction of metal atoms displacement in hydrogenous and hydrogen induced isostructural phases only by deformation distortions of a lattice because of implantation of nonmetal atoms. The matter is that "powef' of the distortions caused by larger N atoms is much more than "power" of the distortions caused by hydrogen atoms [16]. Therefore it is difficult to expect that presence of hydrogen in a lattice can change a direction of metal atoms displacement. Apparently, the observed change of a sign of metal atoms displacement may be explained by various character of chemical bond of nitrogen and hydrogen with metal atoms. According to [17], chemical bonds in Me-N systems are realized so that nitrogen atoms send a part of valence electrons to the conductivity zone of the metal, and thus nitrogen and metal atoms own common valence electrons. It is obvious that between positively charged frames of Me and N atoms repel each other. According to [18], hydrogen atoms are donors as compared to metal atoms. If this is still true in presence of nitrogen, metal atoms become negatively charged and will be attracted to nitrogen atoms. Hence the direction of metal atoms displacement in hydrogenous phase will be the opposite to that of metal atoms in HIPh.

Hydrogen thermoemission in crystals can be explained as relief of potential wells in crystals (Fig. 5). Heavy atoms and hydrogen atoms have different initial wells. Hence to overcome the potential barrier less energy is required for hydrogen compared to heavier atoms.

Рис. 5. Рельеф потенциальных ям атомов в твердом растворе ZrNxHy Fig. 5. The relief of potential

wells of atoms in sol. s. ZrNxHy

Conclusions

1. At low-temperature (T = 375° C) high-vacuum evacuations of hydrogen out of lattice of the solid solution ZrN043H0.38 the new, hydrogen-induced a'-phase (HIPh) Zr2N086 (sp. gr. P 3m1);was obtained that is absent in the phase diagram of Zr-N system; it is impossible to obtain it by traditional methods.

2. During formation of HIPh two observations are made: isotropic compression of the crystal lattice and change for metal atoms displacement directions. Apparently, such effects may be explained by various characters of chemical bond of nitrogen and hydrogen with metal atoms.

3. At rise in temperature of H evacuation two more modifications of the same compound (Zr2N0.86) were obtained but they have somewhat different structure: at T = 450-650° C it is the orthorhombic ordered a''-phase (sp. gr. Pnnm); at T = 750-800° C - disordered hexagonal a-phase (sp. gr. P63/mmc). All the three modifications: a', a'' and a are genetically connected to the initial phase but do not contain hydrogen.

4. Between the found modifications there are reversible and nonreversible phase transitions; conditions and schemes of the transitions are presented.

5. It is established that it is possible to produce the unique metastable phases (HIPh), which do not contain hydrogen but keep the structure of initial phase as "frozen", by means of evacuation of hydrogen out of a lattice of some solid solutions of the Zr(Ti)-N-H system using special regimes of vacuum heat treatment. It shows additional opportunities of obtaining new compounds with defined service characteristics by hydrogenating and dehydrogenating.

Acknowledgements

The work was carried out at financial support of the Science and Technology Center in Ukraine, project № Uzb-131(j).

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