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A new model which describes the peculiarities of the phonon spectra of homo- and heterodesmic crystals is developed in the present paper. A salient feature of the model is that the specific properties of the heterodesmic compounds (existence of large covalent groups) are considered explicitly by inclusion of the corresponding terms into the dynamic equations. Numerical calculations of the phonon spectrum for the model heterodesmic crystal show a large gap (more than two orders of magnitude) between the optical and acoustic branches. Large variations of the non-radiative transition probabilities in doped homo- and heterodesmic crystals are explained by the effective lowering of the phonon state density in the latter case. Application of the model developed to the comparative studies of CeSc 3 (BO 3 ) 4 :Cr 3+ and LiSrAlF 6 :Cr 3+ crystals resulted in good agreement with available literature data.

Absorption and luminescence spectra and luminescence kinetics of Yb have been measured in crystals of Yb,Er:LaSc3(BO3)4 (LSB) for a wide region of variation of activator concentration. It has been established that an effective absorption cross-section of Yb3+ at the maximum of 982 nm is equal to 1.16 × 10−20 cm2. Effective absorption and emission cross-sections of Er3+ at generation wavelength of 1555 nm are equal to σ ab =4.4×10 −21 and σ em =5.2×10 −21 cm −2 , respectively. 2 F 5/2 Yb 3+ → 4 I 11/2 Er 3+ energy transfer for dominating luminescence centers in Yb,Er:LSB is described satisfactorily by a dipole–dipole interaction with C DA =4.16×10 −39 cm −6 . Yb–Yb energy migration is very efficient and described by a hopping mechanism with C DD =3.5×10 −38 cm −6 , the quantum efficiency in Yb0.15 (6.7 × 1020 cm−3), Er0.015 (6.7 × 1019 cm−3):LSB crystals has achieved 98%. Investigations of Yb,Er:LSB crystals have shown them to have necessary combination of properties to get an effective generation of 1.5 μm eye-safe radiation with laser diode pumping.