Polarized Raman spectra of oriented monoclinic single crystals of α-Bi 2 O 3 have been obtained at temperatures of 20-650 K in the region 15-3500 cm −1 . The assignment of the Raman bands to the A g and B g modes was given. The photoinduced reduction of the Raman intensity was observed after heating a crystal above 500 K, and cooling it down to room temperature, under laser irradiation. A lattice dynamical analysis based on two-particle interatomic potentials and a rigid-ion model was performed. These potentials were consistent with the Raman and IR data, besides which they showed a stable dynamics throughout the Brillouin zone, and yielded the minimum of internal forces and pressures in the crystal lattice. The phonon dispersion curves, and the projected and total densities of phonon states were calculated.

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The polarized hyper-Raman spectra of a calcite crystal in different scattering geometries are obtained. Two silent A1u-vibrntions a t 227 and 1085 cm-l as well as infrared active vibrations are found. The upper branch of mixed A2,-poIaritons and their resonance with E,-vibrations is investigated. The calculation of mixed polariton dispersion curves is presented.IIonywm,r nonRpmooairnbre cnewrpbr r~n e p~~o w B~i~a~~~o~r s o r o paccemms K~M C T~J I J I~ 1 i ; l J l L L~I I T~~ npa paamiwmx reo3teTpmx p a c c e m m . Iipoixe nr~rr~rir xmtonmbrx ~o n e 6 arrrrii o6rrapyirccrm nnc JIMAEIH ,,wmhIx" &,-I;one6amii 227 rI 1085 em-'. HccneAosaiia ncpsrrris UCTBL cmmarnmx A g u -n~n~p~~~~~~~ II peaona~rc ~T E I X ~O~R~~T O E I O R c E,-Rone-lI1>HBCJClI ~laCrlC?T AI4CnepCr/IOIIHI~IX KpIiBLIX CRICUlaHIIbIX IIOJfHpEITOHOB. In trodnctionItecently the hyper-ltaman scattering in some crystals mas studied [l to 51. The interest in the hyper-ltainan scattering as an independent spectroscopic method has grown over the last few years becaiisc of the ability to allow the observation of silent modes which is inaccessible by other methods. Silent vibrations forbidden in the Itainan scattering and Ilt-absorption may be active in hyper-Raman spectra due to selection riilcs [GI. For example, two silent vibrations of B1,-sysmmetry at 113 and 406 cn1-I were observed in a rutile hyper-Itaman spectrum 143. Dipole vibrations arc always active in the hyper-Raman scattering. Therefore, the study of polaritons in both noncentrosymmetric and centrosymmetric crystals is possibleThe calcite vibration spectrum was carefully studied by different methods: 11%-absorption and reflection [8], Raman scattering [9], hyper-Raman scattering [3, 101 inelastic neutron scattering [11], theoretical calculations El1 to 131. A small part of the calcite hyper-Kaman spectrum in the frequency region below 300 cm-' was studied and only some IR-active modes were found [3, 101.In the present paper we study the hyper-Raman scattering by phonons and polaritons in a CaCO, centrosymmetric crystal. Some polarized spectra in the frequency region of fundamental calcite vibrations (0 to 1600 cm-l) were obtained and two silent modes were observed, Besides, the polariton hyper-Raman spectra associated with Kaman inactive mixed A*,-vibrations were studied.[5, 71. ExperimentalHyper-Raman spectra were excited by the polarized light of a YAG laser (mi = = 9396 cm-l) and were detected by a niultichannel photoelectric instrument described previously [14]. The power density of the exciting light did not exceed lo8 W/crna inside the crystal. I n the 90" and 180" hyper-Raman scattering geometries t,he laser 142092 Troitsk, Moscow Region, USSR.

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