Abstract
Abstract Electron paramagnetic resonance (EPR) of Mn2+, Ni2+, and Cu2+ in magnesium fluosilicate hexahydrate (MgSiF6.6H2O) crystals has been studied at X- and Q-band frequency in the temperature interval 77 ÷ 400 K. At cooling of crystals with natural abundance of 3dn-ions, at T i1 = 370 ∓ 0.5 K, T i2 = 344 ∓ 0.5 K, T c = 296 ∓ 0.3 K structural phase transitions of the second and the first (the latter two) order were observed. The transition at T i1 is of paraphase-incommensurate phase type, and up to T i2 the form of Ni2+ and Mn2+ EPR lines can be well described in the limit of plane-wave modulation of lattice displacements. At T → T i1, the amplitude of the trigonal modulation decreases according to the power law with a critical exponent β = 0.35 ∓ 0.02. Below T i2 up to T c the Mn2+ line shape was analyzed using the structural soliton model. At T → T c, a succession of step-wise changes in the slope of Mn2+ line shape parameter curves which are less significant than those at T i2 is observed. These step-wise discontinuities in all samples occur at practically the same amounts of line shape parameters. The Cu2+ ions affect T i2 to the greatest extent if compared to other investigated admixture ions. At T c crystals undergo an improper ferroelastic first-order phase transition with the unit cell doublet.
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