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​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 

§ 3.4 polymorphs in the family A₂BX₄ with -K₂SO_4.

  It follows from our data, the compounds Rb₂ZnBr₄ (see § 4.1), Cs₂ZnJ₄, K₂ZnBr₄ [125] and, presumably, (NH₄₎₂ZnJ₄ and Rb₄ZnJ₄ and others may crystallize or relax to more stable crystal structure, described by the space group P2₁/m (type structure Sr₂GeS₄). Consequently, the structure type β-K₂SO₄ in these compounds are metastable. As you know, many of the compounds A₂BX₄ also crystallize into structures in space group Pnma or P2₁/m, and   few of them reliably or supposedly presented simultaneously these two structural modifications [86]. In Table 3.2, the compositions are A₂BX₄ in which: 1) the only observed structure Pnma; 2) was observed structure Pnma and P2₁/m P2/m; 3) was observed only structure P2₁/m.

  My can assume that marked us polymorphic transition α-A₂BX₄ ↔ β-A₂BX₄) is characteristic for the various compounds and can be implemented   for other compounds shown in Table A₂BX_4. However, significantly different relaxation times of the structure to the structure and observed us for Rb₂ZnBr₄ (weeks), and for K₂ZnBr₄, Cs₂ZnJ₄ (several minutes), indicate that polymorphic transition may be unrealized in normal conditions. However, we know of no data on detailed studies of the conditions of the transition between these modifications, although these effects were observed and studied in other crystalline objects, and in particular in disordered polymers and molecular crystals with cis-trans isomeric  structure chains [43].

According to our research and NMR data on the measurement of the heat capacity [77] in all the compounds A₂BX₄ grown from aqueous solutions or have been in contact with the atmosphere is always a small amount (less than 2%) loosely bound water. On this basis, we can assume that the transition β-α is carried by surface orientational ordering of β-α structure in the dipoles of water molecules. Comparison of the spin relaxation times for the α and β structures indicates that dynamically disordered metastable β structure converted into a more structurally ordered dynamically stable structure.   The possible existence of crystal structures with varying degrees of dynamic stability is predicted by mathematical methods of stability theory for systems with interaction ANNN      [104, 130].

   As follows from these studies in some symmetrical classes of such systems are possible bifurcation from stable solutions with conditional period of the structure N-1 to dynamically meta-stable   метаустойчивымstates with N = 1,2,3,4 followed by higher order bifurcations to large periods and increasing the degree of instability. How shown in [104] cases N=3 and N=4 and N=2 at some parameters of interatomic interactions are special, because realized only through the bifurcation реализуются some dynamically unstable solutions with N=1 (which we can compare the structure Rnma). In the vicinity of a line bifurcation with different solutions in different periods are mixed and separated by raising away from this line.

This approach can, to some extent justify the presence of a dynamically more stable modifications under consideration for compounds which space group P2₁/m has no common subgroup communication with the space group Pnma. Type β-K₂ZO₄

Table 3.2.

Chapter 4. Sequence of phase transitions in Rb₂ZnBr₄ at atmospheric and high hydrostatic pressure.

 In this chapter we present the results of studies of phase transitions in Rb₂ZnBr₄ NQR ⁷⁹Br, ⁸¹Br at atmospheric and under high hydrostatic pressure. External exposure is the factor that allows you to get a fundamentally new information about the sequence of phase transitions, in addition to clarify the issues in the form of the NQR lines incommensurate phases.

 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