Incommensurate phases in the improper ferroelastic MgGeF6·6H2O:Mn2+ studied by means of EPR

Abstract

The results of an EPR study of the inhomogeneous phases existing in thetemperature interval TC = 311.0 ± 0.3 K < T < Ti1 = 403 ± 0.3K in improper ferroelastic crystals of MgGeF6 · 6H2O:Mn2+are presented. On the basis of the analysis of the temperature and angle dependences of theexperimental parameters and numerical calculations, the conclusion has been drawn thatat Ti1the crystals considered undergo a transition to a structurally modulated phaseand the order parameter of this transition may be the angle of the Mg[H2O]62+octahedra rotation around the crystal C3-axis.From Ti1to TCthe modes of the modulated phase follow according to a completelyclassical scenario for incommensurate crystals: the origin ofthe incommensurate structure with plane-wave modulation atTi1,the appearance of structural phase solitons below Ti2 = 380 ± 0.3K and decrease of the soliton density to values of ≈ 0.85 as the temperaturedecreases to T ≈ 360K. Below that temperature the soliton density is almost unchanged down toTCand, to successfully describe the experimental spectra, the effect ofvariation of the spin–lattice relaxation rate over the spectrum has alsobeen taken into account within the temperature interval considered. AtTC thecrystals investigated undergo a first-order improper ferroelastic phase transition into amonoclinic phase with unit-cell doubling. A comparison with results on MgSiF6 · 6H2Ofrom a previous investigation and a theoretical analysis of the structure evolutionin these crystals are also presented.