SHEMETOV YEVGENY. STUDIES OF PHASE TRANSITIONS IN THE UNITED A2BX4 STRUCTURE β-K2SO4 METHOD NUCLEAR QUADRUPOLE RESONANCE
English abstract § 1.1 § 1.2 §1.3 § 1.4 § 2.1 § 2.2 §2.3 §2.4 § 3.1 § 3.2 § 3.3 § 3.4 § 4.1 § 4.2 § 4.3 § 4.4 Reference Template pdf abstract
CHAPTER 3. The sequence of phase transitions in compounds A2BX4 (X = J).
In this chapter we provide experimental data (the study of phase transitions in the compounds of Cs2CdJ4 (CCJ); Cs2ZnJ4 (CZJ) and (NH4)2ZnJ4; involving mutually complementary methods, the main of which is the NQR try organize and discuss these data.
§ 3.1. NQR study sequence phase transitions in Cs2CdJ4.
Initially, the synthesis and study NQR 127J compound Cs2CdJ4 were taken in 1983.laboratory IP SB AS USSR diplomnitsey Skripichnikova V.V. [76] under the leadership of A.K. Moskalyov. In the available samples revealed the presence of two phase states, which were attributed to crystalline hydrates Cs2CdJ2-H2O and the mixed state of Cs2CdJ4 + Cs2CdJ4-nH2O. Get resilience nonhydrating connection failed, therefore the studies were limited supervision absorption signals NQR 127J at 770K and the temperature dependence of the dielectric constant e(T) and conductance G(T). By anomalies in the temperature dependence was assumed the existence of phase transitions at T+1 = 1870K , T+2 = 2070K , T+3 = 2310K with a temperature hysteresis ΔТ ≈140K .
In 1986 year studies were continued in [139]. Crystals Cs2CdJ4 were grown from an aqueous solution by the Bridgman method. In studies of NQR 127J for the transition (1/2↔3/2) at a temperature of 77 K, we observed three NQR signal from the samples grown from solution. After annealing the samples at temperatures above 400K, was observed in all samples (at 77 K) 16 lines in the spectrum of the quadrupole resonance.
Studies have shown that under various conditions are realized two different crystal modifications of a substance Cs2CdJ4 - α and β. Modification of α - Cs2CdJ4 is stable in conditions of high humidity and changes in modification -Cs2CdJ4 under forced drying of samples, for example, after warming the crystal above 420K. When the thermally induced in the α-β transition was not observed decrease in the weight of the sample (within 2%) [77], and the NMR data indicated the absence of crystalline bound water in the -modification (within 2%). On this basis it was concluded that the α-phase is a structural modification of the polymorphous compound Cd2CsJ4. β-modification is metastable and the storage of samples under conditions of high atmospheric humidity (h ³ 55%), during the hours £1-20 goes into α-form.
NQR studies were performed in the temperature range 77--4500K using the optimal matching [78-81].
To a temperature of 1300K were measured in liquid thermal chamber with temperature stabilization 0.10K. From the α-form was not observed any anomalies in the NQR temperature range studied. Temperature dependence of the frequency of β-Cs2CdJ4. presented in Figure 3.1. The data clearly indicate the existence of three phase transitions at TC1 = 3350K ; TC2= 2600K and TC3 = 1840K. . In the high-temperature phase observed three spectral lines that usually characterizes the connection β-K2SO4-type structure and phase symmetry D162h (Pnma). However, above TC1 signals are detected only by the decay of free precession. At TC1 NQR signals disappear and reappear only in the vicinity of TC2, where their spectral form is typical of the incommensurate structure. On this basis it was concluded that in the temperature range 335-2600K has incommensurate phase. In the G1 phase observed four NQR signal approximately equal intensity. Phase transition from the phase Jc in the G1 phase accompanied by a slight temperature hysteresis 20K (see inset in Figure 3.1). When approaching TC3 intensity NQR lines from the G1 phase and decreases sharply below 184K appears 16 spectral lines, the intensity of which increases with decreasing temperature. At 770K all 16 lines have equal intensity. Thermal hysteresis in the transition G1 ↔ G2 is absent and there is coexistence of phases G1 and G2 in the temperature range 20K .
From a modification in the temperature range from 3900K to TCA-observed three NQR lines 127J, marked on Figure 3.1 in broken lines. The ratio of the intensities of these lines "1:2:1. Stroke frequency (T) is linear with ∂nI/∂T = 1,5*10-2 MHz /0К, ∂nII /∂T = 1,18-10-2 MHz /0К, ∂nIII /∂T = 5,4*10-3 MHz /0К. Relaxation measurements β-Cs2CdJ4 showed that phase P spin relaxation times are at the limit of time resolution and equipment up for TQ1 <50μs and TQ2<10μs this phase NQR signals are recorded only by induction. NQR signals in the absence of phase Jc can be explained by a decrease in time TQ1 and TQ2 abroad temporal range of the spectrometer. In the G1 phase of the value of the spin-spin relaxation time TQ2³ 10μs.
Figure 3.1. The temperature dependence of the NQR frequencies 127J in CS2CdJ4
This, along with a uniform contribution irregularities present contribution to NQR signals. However, the spin-lattice relaxation time is still below the threshold of measurement TQ1<50μs. Figure3.2. the temperature dependence of the time TQ1 and TQ2 near TC3. With increasing temperature of the G2 phase is observed critical shortening of these times. In the absence of thermal hysteresis G2↔G3 and observation of phase coexistence in a narrow neighborhood of T-TC3 ≈> 2K, it was assumed that the phase transition TC3 is close to the tricritical point. Phase transitions of β-modification were also investigated methods us colorimetry, electro-optics [78.139].
According to X-ray diffraction data given NQR sequence was established as follows symmetry transformation [79]:
_
Pnma (Z=4) Jc P21/n (Z=4)P1 (Z=8)
In the phase system was found Jc incommensurate satellite reflections with qδ varying from 0,25* to 0,24* with decreasing temperature [78].
α -modification of the Cs2CdJ4 was identitsirovana Center structure P21/m (Z=2). Thus NQR relaxation measurements indicate its dynamic stability (TQ1=>1ms, TQ2>200s) compared to -form.
English abstract § 1.1 § 1.2 §1.3 § 1.4 § 2.1 § 2.2 §2.3 §2.4 § 3.1 § 3.2 § 3.3 § 3.4 § 4.1 § 4.2 § 4.3 § 4.4 Reference Template pdf abstract