A novel data on Ag 5sbs 4 and cupbsbs 3 probed by antimony NQR spectroscopy Текст научной статьи по специальности «Физика»

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Volume 15, 2013 No. 1, 13101 - 7 pages

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A novel data on Ag5SbS4 and CuPbSbS3 probed by antimony NQR spectroscopy"

A.Yu. Orlova 1 *, R.R. Gainov1, A.V. Dooglav \ I.N. Pen'kov 2

1 Institute of Physics, Kazan Federal University, Kremlevskaya, 18, Kazan 420008, Russia

2 Institute of Geology and Petroleum Technologies, Kazan Federal University, Kremlevskaya, 18,

Kazan 420008, Russia

*E-mail: [email protected]

(Received December 13, 2012; accepted January 10, 2013)

Investigations of Sb-based chalcogenides, stephanite Ag5SbS4 and bournonite CuPbSbS3, have been performed by 121,123Sb nuclear quadrupole resonance (NQR). In stephanite a phase transition at 140 K and internal diffusion motions with an activation energy of 0.29 eV have been experimentally detected. The analysis of experimental results for bournonite revealed two crystal-chemical positions of Sb in the unit cell with distinct local symmetry. The NQR frequencies v and line-widths Av data indicate that Sb(A)S3 complex has almost axial symmetry, but Sb(B)S3 complex is substantially distorted.

PACS: 76.60.Gv, 91.60.Ed, 91.60.Tn.

Keywords: NQR spectroscopy, chalcogenides, stephanite, bournonite, crystal chemistry, transport properties

1. Introduction

Multicomponent silver and copper chalcogenides have received much attention during recent years due to their important technological applications. These compounds have intriguing optical, electric and ferroelectric properties, as well as ionic conductivity, etc. [1, 2]. To explain physical properties of chalcogenides an exact knowledge of the crystal structure of solids state materials is necessary. In many cases NQR spectroscopy provides detailed information about static and dynamic properties of solids. NQR spectroscopy employs nuclei having a quadrupole moment, such as Cu, Na, Co, As, Sb, Bi, and others [3, 4]. NQR experimental results and their analysis highlight the details of ionic arrangement, phase-structural transformations, exchange interactions, internal movements, etc. [5].

In this report we present some results of investigations of two complex sulfides: stephanite Ag5SbS4 and bournonite CuPbSbS3 by 121,123Sb NQR.

2. Crystal structure

The common feature of the studied chalcogenides is a complex composite structure based on isolated pyramidal groups of SbS3. Stephanite, Ag5SbS4, has the centered orthorhombic unit cell with space group symmetry Cmc2i and contains single Sb atom, forming the SbS3 pyramids (Fig. 1). The SbS3 pyramids are connected only via Ag atoms [6]. In contrast to stephanite, bournonite, CuPbSbS3, has the unit cell with space group symmetry Pn2im and contains two crystallographically different Sb atoms in a unit cell: Sb (A) and Sb (B) (Fig. 2). Both Sb (A) and Sb (B) atoms form SbS3 pyramids, which are connected via Cu or Pb atoms [7, 8].

f This paper material was selected at XV International Youth Scientific School "Actual problems of magnetic resonance and its application", Kazan, 22 - 26 October 2012. The paper was recommended to publication in our journal and it is published after additional MRSej reviewing.

Figure 1.

The crystal structure of stephanite Ag5SbS4. The trigonal pyramids SbS3 are formed by the Sb atoms (at the only crystallographic position in the unit cell) surrounded by three S atoms. The figure was obtained with the BS Tools software [9].

Figure 2. The crystal structure of bournonite CuPbSbS3. The trigonal pyramids SbS3 are formed by the Sb atoms surrounded by three S atoms. Two crystallographically different Sb (A) and Sb (B) positions are present. The figure was obtained with the BS Tools software [9].

3. Theoretical background of NQR

The Hamiltonian describing the interaction of the nuclear quadrupole moment eQ with the local electric field gradient (EFG) created by the on-site and surrounding electric charges can be written as [5]:

HQ= TeQh) ^"1 I+1)+0^(7+2 +1-)}, (D

where Vz is the largest component of the EFG tensor;

V - V

xx yy /1 \

V = —^^ (2a)

zz

is the asymmetry parameter showing the deviation of the EFG symmetry from the axial one; the value of n lies in the range [0; 1].

The EFG components satisfy Laplace equation

Vxx + Vyy + Vzz = 0. (2b)

As follows from (2a) and (2b),

Vyy =-0.5fo + l)Vzz, (3a)

V,x = 0.5^- l)Vzz. (3b)

One of the goals of an NQR measurement is to determine the quadrupole coupling constant eQVzz and the asymmetry parameter n, which contain information about the environment of the nucleus. The magnetic dipolar transitions obeying the usual magnetic dipole selection rule Am = ±1 can be observed in the NQR experiment. The corresponding NQR frequencies depend on two parameters - eQVzz and n. In the case n < 0.2, approximate dependences of the NQR frequencies on the asymmetry parameter are known [5]. For example, for I = 7/2:

k±7/2^±5/2 = eQVzz(l - 0.09V -0.01V), (4)

_ 2

'±5/2^±3/2 _ 14

—eQVzz (l - 0.516^2 +1.048^4 ), (5)

K±3/2^±1/2 14

— eQVzz (1 + 3.42V - 4.01V). (6)

4. Experimental part

Two natural powder minerals, Ag5SbS4 and CuPbSbS3, have been studied. Samples were kindly granted by the Geological Museum, Kazan Federal University. Phase homogeneity and structure were confirmed by X-ray diffraction analysis at room temperature. The NQR measurements were performed using 121Sb nuclei (nuclear spin I = 5/2, natural abundance 57.25%, gyromagnetic ratio y/2n = 10.188 MHz/T, nuclear quadrupole moment Q = -0.543 b) and 123Sb nuclei (I = 7/2, 42.75%, y/2n = 5.517 MHz/T, Q = -0.692 b). Pulsed NQR spectrometer with quadrature detection has been used.

5. Results and discussion

The 121,123Sb NQR spectra of Ag5SbS4 and CuPbSbS3 are shown in Fig. 3 [10]. The number of NQR lines is defined (i) by the amount of crystallographic nonequivalent positions of quadrupole nucleus in the crystal structure, (ii) by the magnitude of nuclear spin I and (iii) by the number of naturally available isotopes of quadrupole nucleus. Antimony has two abundant isotopes: 121Sb (57.25%) with nuclear spin I = 5/2 and 123Sb (42.75%) with nuclear spin I = 7/2. So, for I = 5/2 two NQR lines can be observed, corresponding to transitions mI = ±1/2 ^ ±3/2 and ±3/2 ^ ±5/2. For I = 7/2 three NQR lines can be observed, corresponding to transitions mI = ±1/2 ^ ±3/2, ±3/2 ^ ±5/2 and ±5/2 ^ ±7/2. Thus, the single antimony crystal-chemical position in the structure of the material under study yields

121 123 121

five NQR signals. The , Sb NQR spectrum for Ag5SbS4 consists of five lines (two lines for Sb

123 121 123

transitions and three lines for Sb transitions), whereas , Sb NQR spectrum for CuPbSbS3 contains ten lines. These results are in good agreement with the crystallographic data. The observed number of Sb NQR lines confirm that Ag5SbS4 has only one crystal-chemical site of antimony, but CuPbSbS3 has two nonequivalent antimony positions.

500

400

300

200

> 100

CO

c 0

<D 500

c

400

300

200

100

0

■ I i I i I i I i I i I i I i I i I i I i I i I i I i I i I i I i I i

stephanite

Ag SbS _ 121Sb

AgsSbS4 -

V-V

hournonite CuPbSbS3

i23Sh (B)

i2iSb (B)

J23S_b ÎAL

121Sb (A)

ii

30 40 50 60 70 80 90 100 110 120 Frequency (MHz)

Figure 3. The 121,123Sb NQR spectra of stephanite, AgsSbS,, and bournonite, CuPbSbSs. The number of NQR lines indicates that Sb atoms occupy only one crystallographic position in Ag5SbS4, whereas in CuPbSbS3, two positions (referred to as A and B). For details, see text.

Stephanite

The temperature dependence of 123Sb NQR frequency v and NQR line-width Av for Ag5SbS4 are shown in Fig. 4 (transition ±1/2 ^ ±3/2) [11]. Generally, the NQR frequency decreases with increasing temperature without any significant anomalies. According to the Bayer model, such behavior corresponds to the averaging of the EFG owing to a temperature-induced increase in the amplitude of lattice vibrations [12]. However, we point out the weak change of the slope in the temperature dependence of v at about 140 K. NQR line-width, Av, has similar behavior: Av decreases with increasing temperature. As one can see in details, there are two bends at 140 K and 250 K on the curve. It is important that the line-widths are not larger than 90 kHz at all temperatures. This rather small value signifies that stephanite represents a high-ordered structure though it is rather complex. More detailed information can be obtained from the temperature dependence of asymmetry parameter n and Vxx component of EFG (Fig. 5).

These parameters were calculated using formulas (3)-(6). In general, the value of n varies not much (about 10%). This indicates substantial, but not large distortions of umbrella-like SbS3 complexes. As one can see, there is a distinct bend in the n(T ) curve at 140 K, and peculiarities in the behavior of the Vxx at about 80, 140, and 250 K. Thus, all parameters of the NQR spectra exhibit an anomalous temperature dependence. The bending character of the curves is typical for structural transformations of displacement type, which change the configuration of the local field, but do not change the symmetry. Thus, we can conclude that

N

34,5

34,0

33,5

T»140 K

• NQR frequency v o NQR linewidth Av.

i

33,0

>

32,5

S !

'o® V !

ee

O i • i 1

T»250 K „

-/-1

&

!©o(

>i

® ©

W R ft

0,08

0,07

0,06 )

N

0,05 X

s

0,04 >

<

0,03

0,02

0 50

Figure 4.

100 150 200 250 300 350 400

Temperature (K)

dependence of 123Sb NQR ±3/2) and line-width Av in

The temperature frequency v (±1/2 AgsSbS,.

1240

1238

CM

1236 E

>

1234

®

1232 *

X

1230 X >

1228

100 150 200 250 300 350

Temperature (K)

400

Figure 5. The temperature dependence of asymmetry

parameter n and EFG stephanite.

Vxx component ( Sb) in

there is a structural transformation of SbS3 complexes in Ag5SbS4 at 140 K [11]. Similar structural changes have been observed in compounds structurally related to stephanite: proustite Ag3AsS3 and pyrargyrite Ag3SbS3. Later it was found that these changes correspond to phase transitions of second order, which are accompanied by slight structural reorganizations of antimony coordination spheres, leading to the appearance of ferroelectric properties [13, 14].

The temperature dependence of antimony nuclear spin-lattice relaxation rate, 1/T1, can be described very well by the expression T11(T) = aT2 + ^exp(-Ea / T). The first term in this expression corresponds to the second order Raman process; second term reflects the presence of internal movements with the activation energy Ea = 0.29 eV. We suppose that the internal motions in Ag5SbS4 are due to the diffusion of silver ions over the crystal structure. This conclusion is confirmed by the theoretical estimations pointing out to silver ions diffusion [6]. Another evidence of Ag diffusion is narrowing of spectral line (Fig. 4) due to averaging of the EFG distribution by diffusion motion. Interestingly, ion conductivity due to movement of Ag ions was also experimentally observed in pyrargyrite Ag3SbS3 and proustite Ag3AsS3, with an activation energy of 0.4 and 0.42 eV, respectively [15].

Bournonite

The temperature dependence of v for the both antimony positions in CuPbSbS3 shows a normal Bayer behavior, as in the Ag5SbS4. However, in contrast to stephanite, no peculiarities in v(T), n(T) were revealed, which means that there are no prominent structural transitions in bournonite. However, analysis of spectroscopic parameters points to interesting crystal-chemical features of CuPbSbS3.

The NQR frequencies v and corresponding line-widths Av for both A and B positions in CuPbSbS3 at T = 77 K are presented in Table 1. The values of quadrupole constant eVzzQ and asymmetry parameter n have been calculated from experimental data (using formulas (5)-(6)). The obtained values of the asymmetry parameter n for A and B antimony positions (0.5% and ~ 22%) indicate the significant difference in the symmetry of the corresponding SbS3 pyramids. The Sb(A)S3 pyramid has the symmetry very close to axial, while Sb(B)S3 is substantially distorted. It should be noted that the change in n with temperature is negligible for both positions. The average value of the quadrupole constant eQVzz for both positions is about 470 MHz. However, stibnite Sb2S3, structurally related to CuPbSbS3 compound, has the value of eQVzz(123Sb) about 297 MHz for the A position and 397 MHz for the B position [16]. Such difference can be explained by an increase of a Sb-S bond polarity caused by the fields created by metallic atoms (in CuPbSbS3 case, Cu and Pb) [17].

Table 1. The NQR spectroscopic parameters in bournonite CuPbSbS3 at 77 K.

Position Transition v (MHz) Av (kHz) eQVzz (MHz) n (%)

A 123Sb ±1/2 ~ ±3/2 34.275(5) 35.5(5) 479.6(6) 0.5(1)

123Sb ±3/2 ~ ±5/2 68.509(4) 62.3(4)

B 123Sb ±1/2 ~ ±3/2 38.683(5) 210 ± 20 463.2(6) 21.8(5)

123Sb ±3/2 ~ ±5/2 65.453(5) 60.0(5)

The detailed shape of 123Sb NQR spectral lines (transitions ±1/2 ^ ±3/2) for the A and B positions in CuPbSbS3 are shown in Fig. 6. As one can see, the NQR line-width of the B position is larger than that of the A position.

The NQR line-width is a good indicator of the degree of lattice perfection of a crystal under study. As follows from Eqs. (4)-(6), the NQR line-width is determined by the distribution AVzz of the Vzz component of the EFG and the distribution of the asymmetry parameter An. For bournonite, 123Sb NQR line-width is 35 kHz for the site A (transition ±1/2 ^ ±3/2). This is a relatively small value for a natural mineral pointing to a high degree of structural order. However, Sb line-width is 250 kHz for the site B. This indicates that antimony atoms at the site B do not occupy a definite position with respect to their neighboring sulfur atoms in the Sb(B)S3 complexes; in other words, their locations are distributed around the average position. Such distribution Sb is caused probably by lattice

1200 1200

Sb(A)

±1/2 - ±3/2

1000 - - 1000

.—.

c

D 800 - - 800

.Q

r

600 - - 600

>

| 400 - - 400

en

£ 200 - - 200

0 . J L— 0

33,6 34,0 34,4 34,8 38,0 38,4 38,8 39,2 39,6

Frequency (MHz)

Figure 6. The 123Sb NQR spectral lines for the positions A (left) and B (right) in CuPbSbS3 at 77 K (transition ±1/2 ~ ±3/2).

defects, different impurities embedded in the crystal. On the contrary, positions of the Sb(A) atoms are almost the same in all of the Sb(A)S3 pyramids. Thus we can conclude that lattice defects are located irregularly and concentrated mainly in the vicinity of the Sb(B)S3 complexes. This is typical feature for low-symmetry crystals having several nonequivalent atomic positions in the unit cell [16].

In addition, we performed calculations of AVzz and An contributions to the line-widths in conformity with [18]. It was found that Av/v « A Vzz/Vzz « 10-3 for the Sb(A) position; i.e. Av is determined by the distribution of Vzz values. However, Av/v ~ An/n ~ 20-10-3 for the Sb(B) position; in other words, the main contribution to broadening of Sb(B) NQR lines is the distribution of the asymmetry parameter n.

The degree of distortion of coordination polyhedra is defined by interatomic distances and valence angles. X-ray diffractometry tends to average atom positions over all unit cells, both perfect and distorted. Hence, geometrical parameters (interatomic distances and valence angles) can not be always defined within an appropriate accuracy. For instance, both of the SbS3 polyhedra in bournonite are distorted in a similar manner, according to the X-ray analysis [7]. As it follows from our NQR data, sulfur from Sb(A)S3 umbrella-like pyramids and more distant neighbors create almost axially symmetric EFG at Sb(A) site (n = 0.5%), while the EFG symmetry at Sb(B) in Sb(B)S3 complexes substantially deviates from axial (n = 22%). Actually, it was found recently that the Sb-S distances in the A position are almost the same (2.464, 2.467, and 2.467 A), while the Sb-S distances for the position B have different lengths (2.438, 2.463, and 2.463 A) [8]. Our NQR spectroscopic results are in a good agreement with these recent studies. Thereby, high sensitivity of the NQR spectroscopy to crystal-chemical properties of solid state materials allows us to specify and correct the crystallographic data. More detailed studies of bournonite will be presented elsewhere.

6. Conclusions

Two natural minerals, stephanite and bournonite, have been investigated by 121,123Sb NQR spectroscopy. The analysis of obtained experimental results demonstrates high sensitivity and accuracy of NQR to individual features of crystal structure of solids including the local symmetry, structural phase transition and transport properties.

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