Crystal structure and magnetism of Co2−xnixb2o5 pyroborate Текст научной статьи по специальности «Физика»

УДК 537.6;548.7

Crystal Structure and Magnetism of Co2-xNixB2O5 Pyroborate

Michael S. Platunov*

Kirensky Institute of Physics, SB RAS, Akademgorodok, 50/38, Krasnoyarsk, 660036, Aerospace University, Krasnoyarsky Rabochy, 31, Krasnoyarsk, 660014,

Russia

Natalya B. Ivanova

Siberian Federal University, Kirensky, 26, Krasnoyarsk, 660074, Krasnoyarsk State Agrarian University, Mira, 90, Krasnoyarsk, 660049, Russia

Natalya V. Kazak Leonard N. Bezmaternykh Alexander D. Vasiliev Evgeny V. Eremin

Kirensky Institute of Physics, SB RAS, Akademgorodok, 50/38, Krasnoyarsk, 660036,

Russia

Sergey G. Ovchinnikov

Kirensky Institute of Physics, SB RAS, Akademgorodok, 50/38, Krasnoyarsk, 660036, Siberian Federal University, Kirensky, 26, Krasnoyarsk, 660074, Aerospace University, Krasnoyarsky rabochy, 31, Krasnoyarsk, 660014,

Russia

Received 10.01.2011, received in revised form 10.03.2011, accepted 20.04.2011 The high quality single crystals of Co2B2O5:Ni were synthesized. The detail study of crystal structure using single-crystal X-ray diffraction was carried out. The monoclinic symmetry was found (P2i/c space group). The magnetization and magnetic susceptibility measurements have shown antiferromagnetic behavior below TN = 47 K and paramagnetic temperature в = 43 K. The effective magnetic moment per magnetic ion was 3.49 , which points out the divalent and high-spin state of Co and Ni ions.

Keywords: transition metal borates, solid state reaction, magnetic susceptibility, magnetic frustration.

Introduction

The pyroborates with general formula MM'B2O5, where M and M' are divalent ions Co, Mn, Fe, Mg, Ca, Sr are interesting primarily due to their structural, magnetic and optical properties [1-10]. The crystal structure can be of triclinic or monoclinic symmetry with space group P 1(2)

* platunov@iph.krasn.ru © Siberian Federal University. All rights reserved

or P2i/c, respectively. The unit cell contains Z = 2 formula units for triclinic structure and Z = 4 for monoclinic one. The metal ions are placed inside distorted oxygen octahedra sharing edges and form substructures — ribbons. These ribbons are extended along the crystal b-axis and contain two distinct crystallographic sites for the metal ions: one in the border columns (1) and another in the central ones (2). In the heterometallic compounds (M = M') the metal ions occupy the both sites so that these materials are intrinsically disordered. The pyroborate group (B2O5)-4 formed by two trigonal (BO3)-3 groups is the most strongly bonded. The each of five oxygen ions belongs to the octahedron and the pyroborate group simultaneously. The low-dimensional substructures in the form of ribbons and zigzag walls are the common feature with another well studied oxyborates, such as warwikites [11] and ludwigites [12].

The magnetic interaction between the neighbor ions within the same ribbon is mainly due to the super-exchange mechanism, while the ions belonging to the neighbor ribbons interact through the BO3 group and, as a consequence, are weakly bonded magnetically. The homometallic pyroborates with M = M' = Fe, Co, Mn show antiferromagnetic behavior with TN = 70 [13], 45 [14] and 24 K [1], respectively. The spin-flop-like transition at 25 kOe was found in Mn2B2O5. The magnetic properties of the heterometallic pyroborates MnMgB2O5 have been studied in detail and random exchange Heisenberg antiferromagnetic chain (REHAC) model was proposed for magnetic behavior description [1]. The magnetic and optical properties study of the powdered Co2B2O5 was carried out [14]. The high-spin state of Co2+ ions and positive Weiss temperature were found.

In the present work we synthesized the high quality single crystals of Ni — substituted Co2B2O5 and investigated the crystal structure and magnetic properties.

1. Experimental Section

The single crystals of Co2-xNixB2O5 were grown using a flux system Bi2Mo5O12-Li2O-B2O3-CoO-NiO. The saturation temperature was Tsat < 980 °C and the crystallization interval was ATcr > 30 °C. The flux was heated at 1050 °C during 4-6 h and fast cooled down to T « Tsat — (10-12) °C and then it was slowly cooled with the speed of (4-6) °C/day. The growing process was continued by the three days. The crystals were dark pink in color, circa 3mm long, they had an oblique prism shape and good optical quality. After the reaction the product was subjected to etching in 20% aqueous solution of nitric acid. The molar ratio of CoO : NiO in the flux was 1 : 0.15, which corresponds to x = 0.26.

A room-temperature X-ray diffraction analysis was performed on pink prism-shaped single crystal. The measurements were carried out on a SMART APEX II diffractometer with graphite monochromatic MoKa radiation (A = 0.71073 A). The structure was solved using the software SHELXS [15] and refined using the software SHELXL-97 [16].

The magnetic properties were investigated using a commercial PPMS 6000 platform (Quantum Design) on powder sample. The temperature dependences of the magnetization and magnetic susceptibility were measured at an applied field of 500 Oe and 2-300 K. The magnetization isotherms were obtained in the field up to 90 kOe and 2-55 K.

2. Results and Discussion

The crystal data for Co2-xNixB2O5 are summarized in Table 1. The compound crystallizes in a monoclinic structure (P21/c) while Co2B2O5 has a triclinic crystal system [14,17]. The lattice parameters are well agreed with those for Co2B2O5 except for b-parameter, which is twofold.

The Table 2 shows the atomic coordinates and anisotropic displacement parameters. The metal ions (Co, Ni) and B have two sites and O atoms have five sites. These sites are at general

4e Wyckoff position. The Table 3 lists the bond lengths, bond angles, bond valence sums (BVS), distortion indices and electric field gradient (EFG) values for Co2_xNixB2O5.

Table 1. Crystal data and structure refinement

Empirical formula Co4Ni4BsO20

Wavelength 0.71073 A

Crystal size 0.24x0.06x0.12 mm3

Temperature 298 K

Crystal system Monoclinic

Space group P2i/c

Unit cell parameters

a, A 9.2330(22)

b, A 3.1579(8)

c, A 12.3593(29)

a 90.000°

ß 104.183(3)°

Y 90.000°

V A3 349.4(3)

Z 4

Density (calculated) 4.168 g/cm3

20 range (deg) 5-90

Absorption correction Gaussian

Refinement method Full-matrix least squares on F'

2

Table 2. Atomic coordinates (x104) and anisotropic displacement parameters (A x 104) for

Co2-xNixB2O5

Atom Ni(1) Co(1) Ni(2) Co(2)

O(1) O(2)

O(3) O(4) O(5) B(1) B(2)

x 3569 3569 1025 1025 2589 -536 4963 1801 2984 -1682 3553

y

2052 2052 7159 7159 2302 2198 -3052 -2773 701 1525 2766

z

6049 6049 6870 6870 7377 6528 6374 5467 4292 5609 8405

Uii 97.4 97.4 77.4 77.4

33.1

38.2

74.3 70.9 57.6 85.1 100.3

U22 110.3

110.3 86.9 86.9 110.6

109.4

119.2

132.3 117.9

64.4

48.5

U33 105.9 105.9 95.7

95.7

71.3 92

113.2

62.8 77.1 106.7

98.4

U12 -10.8 -10.8 5.2 5.2 18.2 4.5 3.2 -22.3 -24.8 6.2 -22.1

U13 31.7 31.7 30.1 30.1 47.9 61.6

54.4

47.5 9.5 12

81.9

U23 -9.6 -9.6 4.1 4.1 11.6 -6.8 18.3 -6.6 -1.5 -10.3 -28.6

Fig. 1 shows the coordination environments around metal and boron atoms. The composite ion B2O5 is formed by two B1O3 and B2O3 triangles linked by sharing O5 atom. The metal Co/Ni atoms are surrounding by the distorted oxygen octahedra. The structure of Co2-xNixB2O5 projected to the ac-plane is shown in Fig. 2, a. Four (Co,Ni)O6 octahedra, linked by edge-sharing, are aligned in the chain of the (Co,Ni)1-(Co,Ni)2-(Co,Ni)2-(Co,Ni)1 and form the (Co,Ni)4Oi8 units. These units are extended along b-axis and form a ribbon substructure. The Fig. 2, b represents the ribbon substructure along b-axis. The each ribbon represents almost hexagonal lattice of Co/Ni ions. The mean closest interatomic M-M distance is 3.1686(1) A.

Fig. 1. Metal and boron coordination environment in Co2-xNixB2O5. The symmetry labels correspond to those defined in Table 3

Fig. 2. a) Crystal structure of Co2-xNixB2O5 viewed in the ac-plane. b) The ribbon plot showing the hexagonal arrangement of metal ions. The M1 and M2 sites are marked by light and dark octahedra, respectively

We have estimated the valence state of metal (M1, M2) and boron ions by means of the bond valence sum calculation

Z - sij,

Sij — exp

i=1

(R0 - Vij )

b

(1)

Here sij — the bond valence between i and j ions, R0 — the bond valence parameter (BVP)

dependent on the nature of ions forming the ij-pair, b — the constant value 0.37 A, rij is the distance between i and j atoms [6, 18, 19]. The BVP values for Co2+, Ni2+ and B3+ are 1.692, 1.654, and 1.371 A, respectively. The calculations of bond valence sum for B at the triangular coordination and M at the octahedral one were shown that boron and cobalt/nikel are trivalent and divalent, respectively (see Table 3).

The interatomic distances inside BO3 group appeared to be very short. When calculated from the above parameters they are found to be: mean B-O bond lengths are 1.3850(3) and 1.3785(6) for B1 and B2, respectively. The closeness of mean B-O-B bond angle to 120° corresponds to with the trigonal planar geometry [9]. The Co/Ni-O bond lengths range from 1.9891(4) to 2.2973(4) for the atoms at the octahedral site 1 and from 2.0319(4) to 2.1616(4) for ones at the octahedral site 2. From the interatomic distances given above it is seen that the connection forces within the B2O5 group undoubtedly must be much stronger than the other bonds. There exist a large dispersion in the bond angles of both octahedra. The O-(Co/Ni)-O bond angles range from 63.49° to 110.38° for Co1/Ni1 and from 82.37° to 101.75° for Co2/Ni2. The distortion index of (Co1/Ni1)O6 octahedron is three times larger than that of (Co2/Ni2)O6.

The distortions degree of oxygen octahedra MiO6 and M2O6 also can be estimated according the value of electric field gradient (EFG). The lattice sums calculations made for the oxides with the spinel structure have shown that oxygen anions, belonging to the closest coordination octahedra, sufficiently screen the contribution from the next-nearest coordination spheres [20]. Therefore, we limited our calculation by the contribution from the first coordination sphere. In this case the mane component Vzz of the EFG tensor may be calculated as

E3 cos2 в — 1

2e-^-> (2)

where e — elementary charge, в — the angle between the main axis (z) of EFG tensor and the adjacent oxygen ion direction, r — the metal — oxygen distance. The results are given in the Table 3. One can see that site 2 has more symmetric oxygen surrounding, while the Mi O6 octahedra situated at the ribbons edges are exceedingly distorted. This conclusion well agree with those reported for the other isotypic pyroborates such as CoMnB2O5, M1.5Zno.sB2Os (M = Co, Ni).

The intra-ribbon distances Co1/Ni1-Co2/Ni2 and Co2/Ni2-Co2/Ni2 are 3.1786-3.2121 and 3.1526-3.1579 A, respectively. The closest inter-ribbons metal distances Co1/Ni1-Co1/Ni1 and Co2/Ni2-Co2/Ni2 are 4.2069(7) Aand 4.8872(9) A, respectively. The closest distances Co1/Ni1-Co1/Ni1 for coplanar ribbons and for adjacent ones are 4.5316(7) and 4.2069(7) A, respectively.

The temperature dependencies of the ac-susceptibility real part for the different frequencies are shown in Fig. 3. The sharp peak near 47 K and small divergence at low temperatures are visible. According to the M(T) form this magnetic transition can be identified as an antiferromagnetic one. It is in agreement with the results of T.Kawano et al [14]. There is no frequency dependence of TN.

Fig. 4 shows the temperature dependence of the inverse dc-susceptibility measured in a magnetic field 500 Oe. The high-temperature susceptibility is well fitted by Curie-Weiss equation x = C/(T - eCW), with eCW = 43 K indicating on the predominance of ferromagnetic interactions in the system. The FM interaction was found also for the parent Co2B2O5 but it is much weaker (eCW = 7.7 K) comparing Co2-xNixB2O5. The Curie constant C = 1.53 emu-K-mol-1-Oe-1 per magnetic ion. The effective magnetic moment per magnetic ion ^eff = 3.49 . The ^eff value is less than one for the parent compound and points out the divalent and high-spin state of the Co and Ni ions. The substitution of the part of Co2+ (S = 3/2) ions by Ni2+ (S = 1) ones should give rise to the magnetic moment decreasing, as it is observed. All the magnetization isotherms obtained at T = 2 - 40 K are linear. Two of them are shown in Fig. 5. The magnetic susceptibility in the antiferromagnetic state xaf = 2.01 • 10_5/uB/f.u./Oe (T = 2 K). The curves above the magnetic transition are typical for paramagnetic behavior.

Table 3. Bond lengths (A), angles (deg), bond valence sum (BVS), distortion indices and the

main component VZZ of the electric field gradient for Co2_xNixB205

Cobalt (nikel) coordination

Co1/Ni1 - 01 2.0601(4) Co2/Ni2 - 01 2.0950(4)

Co1/Ni1 - 03 2.0407(4) Co2/Ni2 - 01A 2.1616(4)

Co1/Ni1 - 03A 1.9891(4) Co2/Ni2 - 02 2.1003(4)

Co1/Ni1 - 04 2.2199(4) Co2/Ni2 - 02A 2.1185(4)

Co1/Ni1 - 04A 2.2973(4) Co2/Ni2 - 02B 2.1360(5)

Co1/Ni1 - 05 2.1480(5) Co2/Ni2 - 04A 2.0319(4)

04A - Co1/Ni1 - 05 80.898(4) 02A - Co2/Ni2 - 04A 100.731(5)

01 - Co1/Ni1 - 04A 78.755(4) 02A - Co2/Ni2 - 02B 83.876(4)

01 - Co1/Ni1 - 03 104.633(6) 01 - Co2/Ni2 - 02B 90.801(5)

03 - Co1/Ni1 - 05 91.835(5) 01 - Co2/Ni2 - 04A 84.328(5)

04 - Co1/Ni1 - 05 63.493(4) 02 - Co2/Ni2 - 04A 101.751(5)

01 - Co1/Ni1 - 04 81.527(4) 02A - Co2/Ni2 - 02 96.926(4)

04A - Co1/Ni1 - 04 88.688(4) 02B - Co2/Ni2 - 02 84.314(4)

03 - Co1/Ni1 - 04 84.271(5) 01 - Co2/Ni2 - 02 84.420(5)

03A - Co1/Ni1 - 05 110.379(5) 01A - Co2/Ni2 - 04A 83.616(4)

03A - Co1/Ni1 - 04A 83.427(5) 01A - Co2/Ni2 - 02A 82.370(5)

01 - Co1/Ni1 - 03A 101.444(6) 01A - Co2/Ni2 - 02B 90.301(5)

03 - Co1/Ni1 - 03A 103.181(4) 01 - Co2/Ni2 - 01A 95.772(4)

03A - Co1/Ni1 - 04 170.818(6) 01 - Co2/Ni2 - 02A 174.340(6)

01 - Co1/Ni1 - 05 139.662(6) 02B - Co2/Ni2 - 04A 171.772(6)

03 - Co1/Ni1 - 04A 171.600(5) 01A - Co2/Ni2 - 02 174.615(5)

Boron coordination

B1 - 02B 1.3656(2) B2 - 03C 1.3544(3)

B1 - 04D 1.3665(3) B2 - 01 1.3694(2)

B1 - 05D 1.4230(3) B2 - 05D 1.4119(2)

02B - B1 - 04D 128.686(8)° 01 - B2 - 05D 118.472(8)°

02B - B1 - 05D 120.313(9)° 01 - B2 - 03C 124.175(10)°

05D - B1 - 04D 110.996(8)° 03C - B2 - 05D 117.33(1)°

B1 - B2 2.6278(4) B1 - 05 - B2 135.922(9)°

Bond valence sum Co1/Ni1 1.93/1.75

Co2/Ni2 1.96/1.77

B1 2.89

B2 2.95

Distortion index Vzz, e/A3 (Co1/Ni1)Oe 0.045 0.167

(Co2/Ni2)06_0.015_-0.024_

Symmetry transformations used to generate equivalent atoms: A: x, 1+y, z; B: -x, 0.5+y, 1.5-z; C: 1-x, 0.5+y, 1.5-z; D: x, 0.5-y, 0.5+z.

T, K

Fig. 3. The real part of the ac-magnetization for different frequencies. The magnetic field amplitude is 10 Oe

<D

o

3

E

03

350

Fig. 4. The temperature dependence of inverse dc-susceptibility measured in the field H=500 Oe. The straight line is guided by the eye

The intra-ribbon M1-M2 and M2-M2 distances are close to 3 A, therefore the direct exchange in Co2-xNixB2O5 is hardly possible. This situation unlikely is in contrast to the case of ludwigites, where the interatomic distances are close to the same [12]. So only the superexchange interactions are possible. The closest inter-ribbons Ml—Ml and M2-M2 distances exceed 4.2 A.

H, kOe

Fig. 5. Magnetization isotherms

The deviation from the Curie-Weiss law reveals below 150 K, that is much higher the magnetic

transition point TN (Fig. 4). This indicates the strong ferromagnetic correlations in the system.

The fact of large positive value of \0Cw| ~ TN indicates that FM and AFM interactions are

congruous in Co2-xNixB2O5.

The behavior of Co2-xNixB2O5 is typical for antiferromagnet but not for the frustrated

system as it can be expected from an almost hexagonal network of the Co(Ni)1 and Co(Ni)2 ions

in the ribbons (Fig. 2, b). Nevertheless in the case of hexagonal structure the AFM interaction

between the magnetic ions belonging to the one ribbon presently produce the frustrated magnetic

bonds. The spin ordering questing was brought up in the work [3]. Using the maximum-entropy

method for the calculation electron density distribution in the (100) plane the authors proposed

a model of spin configurations in Mn2B2O5 below TN. According to this model, the magnetic

interaction inside the coplanar ribbons is FM, and the AFM interaction develops between the

adjacent ribbons. This model suits for the description of Co2-xNixB2O5 magnetic behavior: one

can suppose that |#CW \ value is determined by FM intra-ribbons superexchange interaction and

TN value is due to AFM inter-ribbons superexchange interaction through (BO3)3- anion.

We estimated the intra-ribbons exchange integral by means of the mean field (MF) prediction

.„ . 2zJS(S +1) , - . . . . , . ,

\"cW \ = -;-, where S is an average magnetic moment per magnetic ion determined

_ 3 k

as S = ((2-x)-3/2+x^1)/2, z denotes the number of nearest-neighboured Co2+ and Ni2+ spins. With z = 2 we obtain J/kB - 9.23 K.

Conclusion

The single crystals of Co2_xNixB2O5 pyroborate were prepared using a flux method. The crystal structure study has shown the monoclinic modification whereas the parent Co2B2O5 has the triclinic crystal structure. The magnetization and magnetic susceptibility study have shown the antiferromagnetic ordering below TN = 47 K. The positive Curie-Weiss temperature d just as in Co2B2O5 indicates the predomination of the ferromagnetic interaction between the magnetic

ions, which is stronger in the Ni-substituted sample. The effective magnetic moment (p,eff = 3.49 ^b ) indicates that the magnetic ions Co(Ni) are divalent and are in a high-spin state. The substitution of the part of Co2+ (S = 3/2) ions by Ni2+ (S = 1) leads to decrease in average magnetic moment per formula unit.

This study was supported by the Russian Foundation for Basic Research (project no. 09-02-00171-a), the Federal Agency for Science and Innovation (Rosnauka) (project no. MK-5632.2010.2), the Physical Division of the Russian Academy of Science, the program "Strongly Correlated Electrons", project 2.3.1.

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Кристаллическая структура и магнетизм пиробората

Михаил С. Платунов Наталья Б. Иванова Наталья В. Казак Леонард Н. Безматерных Александр Д. Васильев Евгений В. Еремин Сергей Г. Овчинников

Синтезированы высококачественные монокристаллы Со^В^О^Мг. Проведено детальное исследование кристаллической структуры с использованием монокристаллического рентгеновского ди-фрактометра. Обнаружена моноклинная симметрия (пространственная группа РЁ^/с [14]). Измерения намагниченности и магнитной восприимчивости выявили антиферромагнитный переход при ТN = 47 К и парамагнитную температуру в = 43 К. Эффективный магнитный момент, приходящийся на магнитный ион, найден равным 3.49 ^в, что указывает на двухвалентное и высокоспиновое состояния ионов Со и Ж.

Ключевые слова: бораты переходных металлов, твердотельная реакция, магнитная восприимчивость, магнитная фрустрация.