CRYSTAL AND MAGNETIC PROPERTIES OF THE NEW COPPER-SODIUM BORATE CuNaB306 • 0.842H20
A. M. Vorotynov"*, A. N. Vasiliev", V. V. Rudenko", O. A. Bayukov", D. A. Velikanova, S. G. Ovchinnikova, O. V. Vorotynovab
a Iiirensky Institute of Physics, SB HAS 660036, Krasnoyarsk, Russia
b Siberian Federal University 660041, Krasnoyarsk, Russia
Received April 2, 2014
A new compound CuNaBsOo • O.842H2O was grown for the first time. Its crystal structure, magnetic susceptibility, and magnetic resonance properties are presented. It was proposed that CuNaBsOo • O.842H2O is a spin-Peierls magnet with the transition temperature Tsp ~ 128 K and a ladder spin structure. The possibility of a structure phase transition at T < Tsp is predicted.
DOI: 10.7868/S004445101409020X 2. SAMPLE PREPARATION
1. INTRODUCTION
Search for and investigations of new materials with specific magnetic and electric properties are among high-priority directions in the development of physics of magnetism and solid state physics. Great interest in exploring new systems is connected with the solution of some fundamental problems of physics, including physics of magnetic phenomena. In particular, compounds of copper ions are very interesting from this standpoint. Cuprates with spin S = 1/2, as a separate class of compounds, are interesting magnets with different magnetic structures, which are characterized by different magnetic dimensionality and low-temperature quantum effects. Metagermanat C11G0O3 investigated by us was the first example of an inorganic spin-Peierls compound fl 3], LiCu202 was a chain antiferromagnet with broken ladder magnetic structure [4], Bi2Cu0.i, a 3D antiferromagnet with four-spin exchange interaction [5], and CuGa204, a spin glass state compound [6]. In continuation of this work, single crystals CuNaB3Oe • 0.842H20 were grown for the first time in the system SrO CuO B2Os Na2B407 H20. The results of the investigation of their magnetic and resonance properties are presented for the first time.
E-mail: S&S& 'fliph.krasn.ru
Single crystals of the compound CuNaB3Oe • 0.842H20 were grown in the system SrO CuO B2Os Na2B407 H20. The technological process can be divided into three stages:
1. The original composition of Sr(NOs)2 (17.3 weight %), CuO (43 weight %), and H3BO3 (39.7 weight %) after grinding was annealed in a crucible during 24 hours at T = 850 °C. Such annealing is necessary for at least three times in order to produce a dark-green polycrystallinc phase.
2. Reagents Sr(N03)2 (4.2 weight %), CuO (10.3 weight 7c), B203 (5.4 weight 7c), and Na2B407 (77.0 weight 7) and the synthesized polycrystallinc phase (3.1 weight 7) were melted in a Pt crucible located in the furnace. After dissolution of the reagents, the temperature was decreased to 740 °C and the melt was mixed up.
3. Then the cooled mixer and the crucible were located in a glass with hot water and dissolution of the melt occurred. After cooling, glass crystals in the form of needles of dark-blue color up to 1.5 111111 in length dropped out.
Table 1. Experimental data and structure refinement parameters
Formula CuNaBaOe • 0.842H20
Molecular weight 232.98
Temperature, Iv 298
Space group P21/c
Z 4
max 57°
a, C, A 3.4924(4), 13.428(1), 11.609(1)
P 96.822(1)°
V, A3 540.6(1)
p, g/'em3 2.863
//, mm-1 4.103
Number of peaks 4929
Independent peaks 1364
Number of peaks with F > 4<tf 1185
h, k, 1 linits ^4 < h < 4, —18 < k < 17, —15 < / < 15
Refinement results
Weight refinement on F2 w = [<r2 + (0.029P)2 + 0.35P]-1, P=(F2 + 2F2)/3
Number of the refinement parameters 111
Rl[Fa > 4<t(F0)] 0.0253
wR2 0.0608
GooF 1.071
The extinction parameter 0.0016(9)
(-V)„,„.,.. o/A3 0.43
(A/>)(„,„. o/A3 ^0.37
(A/v)max 0.00
3. CRYSTAL STRUCTURE
Experimental diffraction data were collected from a prismatic form single crystal with dimensions 0.126 x x 0.124 x 0.374 mm3 by using single crystal A'-ray au-todift'ractoriietor SMART APEX II with a CCD detector (Bruker AXS), MoKQ-radiation. Corrections for absorption are entered on the basis of their calculation by the multi-scan method (SAINT) [7]. Because the initial total formula of the sample was not known, it
was determined in the course of searching for the model and its refinement. As a result, all atoms of the compound were identified and located, including an unexpected water molecule. The positions of nonhydrogen atoms were refined in the anisotropic approximation of thermal fluctuations and hydrogen atoms of water that were allocated from the electron density difference syntheses were further specified in the fixed condition (rider model). It turned out that refinement of the fill factor (sof) of the water position lowers the R-factor
Fig. 1. Stacking of the structural blocks in the CuNaB306 • 0.842H20 crystal
down to 0.2% and leads to the value sof = 0.842 (7). All calculations were performed with the assistance of the complex SHELXTL [8]. Crystallographic experimental data and refinement parameters are presented in Table 1.
The structure is built primarily of endless zigzag chains of VO3 triangles, where links of triangles from atoms Bl, B2, and B3 are repeated with 180° rotation for the triangles. The triangles are connected by shared vertices 01 and 02, and links by 06 vertices (Fig. 1).
The planes passing through adjacent chains are parallel to each other and coincide in orientation with a package of the crystallographic planes (102), deviating from them by ±0.45 A. Because the interplane distance is here equal to 3.141 A, the plane chains are broken into pairs with distance 0.9 A in a pair (Fig. 2).
The metal ions are located only between pair planes with the deviation from the plain (102) about ±0.03 A for Cu2+, and ±0.25 A for Na+ (see Fig. 3).
The copper polyhedron CuOg consists of the oxygen atoms 03, 04, 04% and 05 that are situated at
four vertices (O4 atoms are connected by the symmetry center) and are common with BO3 triangles from a chain pair (see Fig. 1). The Cu-0 distances here vary from 1.919 to 1.939(2) A. The other two oxygen ions 04n and 05m are common with the triangles from a neighboring chain pair. The Cu-04n distance is equal to 2.781(2) A and Cu-05"* is 2.809(2) A. As a result, CuOe polyhedrons are grouped into flat tapes along the a parameter (Fig. 4), connecting with neighbors by four edges each.
In turn, the sodium ion is coordinated by seven oxygen atoms, two of which belong to water molecules (Fig. 5). The oxygen atoms are arranged such that four of them are separated by no more than 0.07 A from the plane shown in Fig. 5. The angle between the normal to the plane and the line passing through the 03i and Na atoms is equal to 5.5°. The distances Na-0 lie in the range 2.332(2) A to 2.612(2) A.
A water molecule is common for two Na ions. The Na-Ow-Na angle is equal to 93.8(1)° at the distances Na-0 2.342(3) and 2.441(3) A. The positions of the hy-
13 >K3T<D, Bbin.3(9)
609
• o
c H
Fig. 2. Position of the planes of the BO3 polyhedra in the structure of CuNaB306 • 0.842H20. Not all atoms are shown
Table 2. Hydrogen link parameters in the
CuNaB306 • 0.842H20 crystal
Ow-H, À H-06, À Ow ... 06, À 0w-H-06
0.97 2.47 3.297(3) 143° 06*
0.97 2.00 2.930(3) 161° 06"
Note, i: -x, y - 1/2, 1/2 - 2; ii: x, 1/2 - y, 1/2 + 2
drogen atoms are determined exactly. They are consistent with the positions determined from the electron density difference syntheses. As a result, there are hydrogen links O-H ... O in the crystal with the parameters listed in Table 2.
4. MAGNETIC MEASUREMENTS AND DISCUSSION
The magnetic measurements of the CuNaB306 • 0.842H20 sample were performed using a SQUID magnetometer at H = 300 Oe in the temperature range 4.2-330 K. The sample consisted of a set of randomly oriented crystals, with a total mass of 85 mg. The temperature dependence of the magnetic susceptibility is presented in Fig. 6.
The magnetic susceptibility has a maximal value at T = 330 K Xmax = 3.1 . 10-6 cm3/g and decreases
monotonically to a minimum at T = 58 K. An increase in the susceptibility occurs with a further decrease in the temperature, and is especially strongly pronounced in the temperature range 4.2K<T<20K.
The susceptibility does not follow the Curie-Weiss law in the entire temperature range, but the very small value of Xmax and its decrease with decreasing the temperature (see below) suggests that antiferromagnetic interactions dominate in the system.
The sharp increase in the susceptibility at low temperatures, in our opinion, is associated with the existence of small amounts of impurities or defects in the sample, which remains paramagnetic down to helium temperatures. With this assumption, the concentration of paramagnetic impurities x « 10-2 was evaluated by least-square fitting of the experimental data in the temperature range 4.2K < T < 15 K by a X = xC/T function, where x is the concentration of paramagnetic impurities and C is the Curie constant for the spin Sen = 1 /2. The fitting curve and its difference from the experimental data are shown in Fig. 6.
The temperature behavior of the dependence of the magnetic susceptibility thus obtained is typical of systems with an energy gap between the ground nonmagnetic and excited states, the so-called spin-Peierls magnets [9-12]. The spin gap evaluation was performed by fitting the low-temperature region (up to T — 150 K) with the equation
X = aexp(-A/fcBT) + X0, (1)
where a is a constant that characterizes the dispersion of the exiting energy [6], A is the energy gap, and Xo is a constant term due to the diamagnetic contribution from the electron shell and Van Vleck paramagnetism. The solid curve in Fig. 6 shows the best fitting results with the parameters a = 8.26 • 10-6, Xo = 4.99 • 10-7 cm3/g, and A/kB = 259 K.
Currently, there are several mechanisms for the formation of a spin-singlet ground state in these systems, namely, the interaction of spin and phonon subsystems [13], the charge [14-17] or orbital [18]
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