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§ 3.3 NQR Studies of phase   transitions in (NH₄) ₂ZnJ_4.

  Synthesis and crystal growth tetrayodtsinkata ammonium author performed using chemically pure NH₄ and ZnJ_2, which were taken in stoichiometric ratio, a method of evaporating an aqueous solution and melt ive.

  In the first case, crystallization is carried out at a temperature of 293⁰K under conditions of low atmospheric moisture   for 10-15 days.

 Depending on the variation of the starting components of various crystallizations were prepared with different crystal habit. When r excess NH₄J grew predominantly hexagonal prismatic crystals with a diameter of 5 mm. With excess ZnJ₂ obtained lamellar intergrowths willows crystallites up to 25 mm in length. To remove any residual crystals were washed with a solution of a chemically neutral fluid. A homogeneous melt of colored iodine compound, which after grinding and annealing was a light gray powder. An x-ray test crystals with hexagonal form the habit of no interest.

Methods of NQR investigations was the same as described above. Temperature measurements were carried out in 380-77K.

Studies have shown [92] that observed at room temperature from6 to 8 NQR lines at 77 and 16 lines of varying intensity. Analie number and intensities of the spectral lines samhles different methods of growth and crystallization allowed to correlate them on two structurally different states. So to a certain structure and spectral lines correspond to type νaI - νa5; and-other structure in lines νI, νII, νIII (in-line). Fig.3.11.

 Later studies were conducted on samples two N1 and N2. From sample Nl at T-290⁰K fixed intensity of the NQR lines at frequencies νa1 - νa5 and structure, and from the sample N2 simultaneously observed NQR signals of a and β-structures in proportion intensely about 1:1. At 77⁰K from both samples were also observed and in-line type NQR, and from sample N1 signals approaching the noise level of the spectrometer. Upon annealing the samples (T = 380K) within 4 hours, an increase in the intensities of the NQR signal type b.

   Spectral line type and traced throughout the temperature range and do not have any anomalies in the temperature dependence of ν(T) (dashed lines in Fig.3.11) and intensities I(T). Stroke frequency NQR lines in the linear type to T) 170⁰K with _I/T = 34,5; _II/T = 14,8; _III/T = -2,0 kHz/⁰K.    Ti - 165⁰K   At a temperature Ti - 165⁰K anomaly in the behavior of the spectra. Peak intensities of the lines ν and νIII diminish lines and νI νIII when passing through the Ti vary in frequency (Fig.3.11). The second anomaly is observed at Tci = l26K, where there is a change in the multiplicity of the spectrum. The low-frequency component is split into three lines with an intensity ratio of ≈1:2:1, and the high frequency components pass into the two NQR lines each. When the transition 1/2 - 3/2 fixed 12 NQR lines ¹²⁷J.

 

 

     

Fig.3.11. Temperature dependence of the frequency spectrum of the NQR ¹²⁷J in (NH₄)₂ZnJ_4.

 

 

 

Fig.3.12.  . Changing the values of the amplitudes A_se and A_i in (NH₄)₂ZnJ₄

 

 

 

  Figure 3.13. The comparison of the NQR spectra of Rb₂ZnBr₄ and (NH₄)₂ZnJ₄

 

Temperature changes in the group of high-frequency lines indicate the temperature of the third spectral anomaly : Т_C2 = 87+2К.

At temperatures 210-370⁰K NQR signals from b state of the crystal are observed as a spin-eco signal A_se, and in the free precession signal decay А_i. With increasing temperature, the amplitude of signal A_se for all of the component ν. Fig.3.12 spectrum decreases, and the amplitude ratio of A_se(I) :A_se(II) : A_se(III), characterized by the ratio of 1:1:3.   ; Amplitude A increase of the noise level and reaching a maximum at T≈310K, fall at higher temperatures. After annealing the sample at T > 310⁰K and subsequent cooling, the quantities A_se and A_i decrease in size, but restored after low-temperature cycles. Such changes in NQR spectra reflect the resonant nature of the observed relaxation of nonequilibrium phenomena. However, due to technical limitations, these affects on this compound investigated is not sufficiently clear.

Based on a comparison with the data obtained by this NQR for compounds Rb₂ZnBr₄ (Fig. 3.13, etc.), Rb₂ZnCl_4, Cs₂ZnJ₄ and other assumption was made that the investigated compounds belong to the structural type β-K₂SO₄ symmetry transformation of the scheme:

165⁰K      126⁰K       87⁰K

D_2h^l6(Pnma)      Jcl      Jc2      C_2v⁹(Pn2₁a)

(Z=4)                            (Z=12)

V=l                                 V=3

The phase between 126 and 87⁰K interpreted as disproportionate, rather arbitrary, and is likely to increase in the period phase q_S = 2/5 or 3/8) and the symmetry Pn21a.

Focus on a comparison of the spectral data obtained by us NQR with similar data for other compounds A₂BHal₄ (Hal = Cl, Br, J) and consider-their compliance with femenologicheskoy model [34], proposed to describe the shape of the resonance lines in the incommensurate systems.

Table 3.1 shows the known NQR data for compounds A₂BHal₄ with an incommensurate phase. These data include: 1) the relative splitting of the extreme components of the spectrum near T_i: _I-III = [_I-_III]/_III ; 2) the values of the temperature coefficients of l/n*∂n/∂T

Table 3.1

β -А2ВHal4	νI МHz	νIII МHz	ΔνI-III %	1/ν*(dνI/dT)	1/ν*(dνo/dT)	νI / ν0     %	TQ1 (ms)	TQ2(μs)
(NH4)2ZnJ4 (AZJ)	87.0	66.0	31	3.9*10-4	-3*10-5	280	150	<100
(NH4)2ZnBr4 (AZB)	66.0	61.5	<26	3.0*10-4	-1.9*10-4	400	160	<100
(NH4)2ZnCl4 (AZ0)	9.8	8.0	<27	2.6*10-4	+1.3*10-4		1000	500
Cs2HgCl4 (CHC)	14.15	11.68	≈22	positive	negative		-	-
K2ZnСl4 (KZC)	9.94	8.08	≈20	positive	negative		600	<200
Rb2ZnJ4 (RZJ)			>19	positive
Cs2HeBr4 (CUB)	93.5	78.6	19	2.3*10-4	+1.6*10-4
Cs2CdBT4 CCCB)	65.9	57.36	15	positive	positive		-	-
Cs2CdJ4 (CCJ)	83.2	71.5	16	7.4*10-5	+ 4*10-5		100	<50
Rb2ZnBr4 (RZB)	64.0	55.0	16	2.5*10-4	0	320
Rb2ZnCl4 (RZC)	9.7	8.3	16	4.4*10-4	+2.4 10-9
Cs2ZnJ4 (CZJ)	79.5	73.9	7	7.4*10-4	+1.5-10-4	200	400	<100
Cs2ZnCl4 (CZC)			8	modulation Cs to NMR [127]
Cs2ZnBr4 (GZB)	62.73	67.62	9	no Jc ?
Rb2CdCl4 (RCC)				structure   Immm

 

 

3) the average half-width of the resonance signals latched by spin-echo above T_i ; 4) The maximum values of the relaxation times T_Q1 and T_Q2 the position I Pnma structure of the compounds studied, where the nucleus of halogens.

Comparison of the relative spacing between the high frequency ν_I and ν_III low frequency NQR lines in the high-temperature phase near T_i, shows that the largest amount of splitting Dν_I-III celebrated for ammonium compounds (up 31%) and the smallest for cesium (up 7%). This fact is explained by difficult from the standpoint of crystal, because radius of the cations NH₄, Gs and close in magnitude and indicates that the cleavage is determined sufficiently dynamic environment nucleus halogen. Some correlation of splitting Dν_I-III   observed size of the halogen atom: for compounds А₂ВHаl₄ same A and B, the splitting decreases with decreasing radius Hal. Due to the fact that the "smaller" Hal atoms polarizability than this may indicate a significant chemical (electronic) contribution to the nature of this splitting. At temperatures close to T_С and 77⁰K major outer frequency splitting observed in average in the compounds with the larger ions. This is in agreement about the importance of data on the dipole-dipole interaction   in these compounds. Nor is there any patterns between types of atoms B (Zn, Cd) and the general character of the temperature dependence of frequency: when changing varieties atom B, is transferring to another compound crystallochemically or structural class.   This implies that the stability of the tetrahedron defined by the electronic environment.

Significant linewidths temperature phase indicate the presence of structural or chemical disorder in these crystals. Suggest the following details in shape. Hal_I spectral distributions (the position of the nucleus in the structure of halogen I Pnma). If in Cs₂ZnJ₄ line shape in the position of Jc faze Hal_I presented low-intensity and high-intensive low frequency component, the compounds of RZB, CHB, CCB, CCJ observed reverse arrangement in the intensities of these components.

In this case, cesium bromide intensely ratio of these components is about 20, and in the second part CCJ only observed when T_C   low intensity as a small wing . As will be described in Chapter 4 of RZB in increased pressure in the Jc phase component intensity ratio discussed approaches unity and becomes comparable with the add-on for AZB and AZC. Since the shape of the line is determined, in particular, the transverse spin relaxation (T_Q2), to a marked difference indicates a different character dynamic environment nucleus deviated from position I (II and III) structure Pnma.   Most clearly seen when compared Cs₂ZnJ₄ with other cesium   analogs.

Temperature dependence of the frequency below T_i unusual and can not be explained by known types (rotary, Vibratory, torsionalny) atomic motions. In the nature of the temperature slope of the NQR frequencies in compounds A₂BX₄ celebrated their graduation large values ​​of sharply positive for high frequency components, to smoothly negative for the lower frequency component of the NQR spectrum. This, along with other data about indicates significantly different character movements and dynamics of structurally nonequivalent nucleus Hal.

When comparing our data with the results of stationary NQR methods [37, 44, 45], there are significant differences. It follows from our relaxation measurements of the T_Q1 and T_Q2 due to the fact that when using the latter can be detected only homogeneous (long-lived), contributions to the absorption signal, whereas the use of spin-echo technique allows us to observe inhomogeneously broadened signals. But even in the latter case there are time limitations instrumental that does not allow enough correct record form broad frequency distributions, as in the case of Cs₂CdJ₄, even with an increase in sensitivity of the method, the signals in the Jo-phase is not fully observable.

Briefly summarizing the results of NQR spectral data presented in this chapter, we can note the following: First, the experimental data obtained under static spectral characteristics indicate a fundamental difference in the nature of their temperature evolution compared to phenomenology   model [34]. Secondly, there is a significant impact on the nuclear dynamics observed frequency spectral characteristics of NQR.

 Time measurement T_Q2 spin-spin relaxation in Cs₂ZnJ and Rb₂ZnBr₄ [103,109] indicate the need to consider the temperature change of the width of the singlet spectral line in Jc phase, whereas in the existing model, the width of individual components of the continuum distribution was chosen to be the width of the line   paraelectric paraphase .. However, NQR data for Cs₂ZnJ₄ (and especially our studies RZB [109]) shows that this assumption is not always competent, since the relaxation times and T_Q1 T_Q2 in Jo phase фазы increase rapidly with decreasing temperature, and the intensity of the two end peaks for all of the observed frequency distributions near Т_i   болееdifferent than an order of magnitude. The observed shape of the spectra are not always the result of hardware distortion, since the relaxation time T _Q2, at least at low temperatures Jo phase, the relation T_Q2 >t₁; t_int; t₂.

Comparing our data with the X-ray data on the measurement of the mean square displacements of atoms in tetrahedral ZnCl paraelectric Rb₂ZnCl₄, where it was established that there are two equal-probable provisions tetrahedra into each other by the libration movement (Fig.3.14) and summarizes literature data on the values of the mean square displacements of atoms in the other A₂BX_4, to some extent consistent with those of the NQR and Raman [42] that the phase transition P-Jc in the compounds studied close to the type of order-disorder. It should be noted that not form symmetric spectral distributions comparable to those observed by us, detected RF   methods and systems of other crystalline, such as in compounds with CDW charge density waves (see e.g. 2H-TaSe₂ [95,96]), used ion conductors with superlattice ordering (Ag₂HgJ₄ [97]), organic-dimensional conductors CH_x-trans [98]   etc. [100]) and even in solid helium-3 not [99]. In all these cases, the observed asymmetry of the resonance line was associated with the anisotropic diffusion of spin or charge against the backdrop of two large-scale spatial or temporal scale. Similar forms registered by EPR in the study of the vibronic interaction A and orbital states in complexes with octahedral symmetry [102]. Overall data analysis allows to chart a path radiospectroscopy explanation NQR experiments in this class of compounds in the direction of electron-nuclear movements in two ambitious timeline.