Journal of Physical Studies 4(2), 202–207 (2000) DOI: https://doi.org/10.30970/jps.04.202 ISOTOPIC EFFECT IN RAMAN SPECTRA OF THE ZnP2 AND CdP2 GYROTROPIC CRYSTALS

S. G. Garasevich, P. I. Korenyuk, A. V. Slobodyanyuk, Z. Z. Yanchuk

National Taras Shevchenko University of Kyiv, Department of Physics,
6 Hlushkova Pr., Kyiv, UA-03022, Ukraine
E-mail: garas@phys.univ.kiev.ua, slobod@office.phys.univ.kiev.ua,
phone: +38 (044) 2664587

A pair of isostructural ion-covalent wide gap semiconductors of zinc diphosphide ZnP$_{2}$ and cadmium diphosphide CdP$_{2}$ transparent in the red region gives a unique opportunity for Raman study of fine peculiarities in lattice dynamics due to such features as strong anisotropy of mechanical properties (easy sliding fracture in $XY$ plane), very strong gyrotropy, very narrow lines even at room temperature and the chance to compare spectra of both crystals with very close physical properties.

Both ZnP$_{2}$ and CdP$_{2}$ crystals may exist in ‟right" and ‟left" enantiomorphic structures corresponding to space groups $P4_12_12$ and $P4_32_12$, respectively, with 24 atoms per unit cell. 61 Raman lines may be expected, taking into account a possible splitting of double degenerated E-modes due to a long range Coulomb interaction or spatial dispersion followed by Raman Circular Dichroism (RCD) which is resulted in verrying Raman intensity for the right and left circular polarized light.

In order to elucidate the degree of participation of metal cations in different modes and to facilitate interpretation of such complicated spectra monoisotope single crystals of zinc diphosphide $^{64}$ZnP$_2$ and cadmium diphosphide $^{114}$CdP$_2$, containing only one isotope of metal $^{64}$Zn or $^{114}$Cd, respectively, were grown by resublimation in quartz ampoules. Their Raman spectra were compared with those of $^{\rm Natur}$ZnP$_2$ and $^{\rm Natur}$CdP$_2$ crystals grown from an ordinary isotope composition correspondsing to natural isotope prevalence. Shifts of lines in ZnP$_{2}$ and CdP$_{2}$ spectra have opposite signs in agreement with the fact that $^{64}$Zn is the lightest isotope and $^{114}$Cd is one of the most heavy ones among the Cd isotopes. The magnitudes of shifts reflect the involving of metal atoms in the corresponding mode and reach 1.3 cm$^{-1}$. High frequency lines are expected due to the motion of P atoms because they manifest no shifts and have a very close position in both the ZnP$_{2}$ and CdP$_{2}$ crystals. The lowest frequency lines correspond to the motion of heavy parts of a unit cell. A relative change of their masses due to isotope enrichment is small and shifts are also small. The largest shifts of lines near 100 cm$^{-1}$ indicate that the metal ions contribute significantly to the corresponding modes. We studied the influence of isotope substitution on $E$-doublets near 100 cm$^{-1}$ in ZnP$_{2}$ and near 93 cm$^{- 1}$ in CdP$_{2}$ that are split due to the spatial dispersion and show strong RCD. We observed only isotopic shifts of these doublets without changes of their splitting, RCD and linewidth. This confirms a collective character of motions in these modes.

PACS number(s): 78.30.-j, 31.30.Gs.

ps pdf