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We report some new results of optical phase conjugation(OPC) studies on three halogenated methanes by stimulated Brillouin scattering (SBS) technique using the second harmonic of pulsed Nd:YAG laser which have not been examined so far.The calculated and experimentally determined SBS gains have been compared. Phase conjugate reflectivities of the order of 70% have been obtained at a pressure as low as 1 bar.

The development of optical phase conjugation using thermally formed gratings in absorbing media, especially the dye solutions, has besides the high efficiencies, many other characteristics which are worthy of study. Here the wavelength depcndence of phase conjugate reflectivity in relation to absorption spectrum of rhodamine-6G and rhodamine-B in water, methanol and ethanol is reported. Further the Bragg condition has been verified for the dominant diffracted beam which shows that the thermal'grating behaves like an optically thick grating.

Light induced refractive index modulations in photorefractive crystals can be used to advantage to obtain optical phase conjugation (OFT) through Degenerate Four Wave Mixing (DFWM). The applications of OPC in various adaptive optical systems are well known. Among the of photorefractive crystals, $LiNBO_3$ has been chosen for the present studies on account of its greater spontaneous polarization and higher refractive index modulation constant.In this paper, we report the studies in pure $LiNb0_3$ and Fe (0.02%) doped sample, using 488 nm, which is towards the absorption edge, whereas the earlier reports were with 514.5 nm. The formation and erasure times of the phase grating and the phase conjugate signal are measured using 488 nm wavelength with the pump beams of power 8 mW and probe beam of power 1.2 mW. In the Fe doped crystal, the formation and decay of the phase grating follow a near exponential pattern. The phase conjugate signal also behaves similarly with a formation time of 840sec and a decay time of 1200 sec. These results are in agreement with the previously predicted values by other workers. In pure $LiNbo_3$ crystal, though the decay of the phase grating follows the exponential form, with a decay time of 2000 sec, the formation of the phase grating is observed to follow a rather different pattern. After a gradual increase in the diffraction efficiency in the first 600 sec, there is a fall during the next 200 sec and an increase thereafter and subsequent fall. The reasons for this type of behaviour is explained on the basis of a dynamic approach to the volume grating that is formed.

A continuous flow cryostat was designed (80-300 K) for the study of dielec. and thermal expansion behavior of solid samples at high vacuum (.apprx.10-5 torr). The temp. up to 150 K was reached by increasing the opening or a control valve near the suction pump. Further opening of the valve for obtaining lower temps. revealed a slow oscillation of temp. between 150 and 125 K. And several methods tried out for avoiding the temp. oscillations are discussed

The Pippard-Janovec relations are derived for correlating the anomalous elastic coefficient and the anomalous specific heat near the phase transitions of ferroelectric crystals. These relations are verified in the case of ferroelectric triglycine selenate crystal.

Introduction Dicalcium strontium propionate (DCSP) undergoes a ferroelectric phase transition at about 28 1.5 K, with the spontaneous polarization occurring along the tetragonal C-axis.1 Takashige et al.2,3 have recently reported ferroelectricity in annealed samples of dicalcium lead propionate (DCLP) in the range 191 K to 331 K. The removal of the inner biasing field by annealing has been known in the case of DCLP3 and DCSP.4 Because of the possible dependence of the inner biasing field on the particle size, a study of the temperature dependence of the dielectric behaviour of the powdered samples of these compounds was undertaken.

The dielectric measurements on diglycine sulphate in the temperature range 5-400 K, show that it is a normal dielectric, unlike triglycine sulphate which is a ferroelectric. The difference in the dielectric behavior of these two glycine sulphates has been explained on the basis of certain structural features derived from a study of their vibration spectra.

$Me_4N)_2ZnCl_4 (TmAZnCl_4)$ shows 6 successive phase transitions at 80-300 K. Polarized Raman spectra of oriented single crystals of $TMAZnCl_4$ and the IR spectra by the $KBr$ pellet technique were recorded at 130-298 K, covering all the known phase transitions,. The frequency of the $Zn-Cl$ stretching vibration increases in Raman and IR spectra with decreasing temp. Three bands are found in the Raman spectrum in the region where Me frequencies usually occur and, in a similar way, 3 bands appear in the IR spectrum at low temp. due to the deformation vibrations of the Me group. The triggering mechanism of the phase transition at low temp. can be due to a coupling of the distortion of $ZnCl_{42}$- groups and the Me deformation, where the motion of relatively free Me rotations is slowed down, favoring 1 equilibrium position below 161 K. The phase transition appears to be an order-disorder one with Me rotational disorder in the high temperature paraelectric phase.

Introduction The alum family of double salts with the general formula M1+M3+(RO4)2.12H2O where M1+ is a monovalent ion (M1+ = K, Rb, Cs, Tl, NH4, CH3NH3, NH3OH3 NH3NH2, etc.) and M3+ is a trivalent metal such as Al, Fe, Cr, V, In, Ga, etc. and R is S or Se, form an isomorphous series and their general features indicate a common cubic space group Pa3. Lipson1 showed subsequently that there exist three different structure types agr, β and γ and the structure of a particular alum is dependent on the radius of the monovalent atom. The agr structure is typical of medium sized ions, the β of the larger ones and the γ of the small Na atom.2 Ferroelectricity has been reported only in alums containing NH4, CH3NH3, NH3NH3 and NH3OH. Their hindered rotations as well as the influence of sulphate group disorder on the dielectric behaviour of alums is still not clear.3 No study of the temperature dependence of the low frequency dielectric constant of some of the alums, particularly those of Cs, Rb and Tl, have been made so The present investigation was undertaken to correlate their dielectric behaviour with their composition and structural differences. Under the same experimental conditions, methyl ammonium and ammonium alums also were studied and compared with the known data.

The Raman spectra of NaLa(MoO4)2 single crystal have been recorded and interpreted on the basis of C4h symmetry. The observed fundamentals (internal and external) have been assigned unambiguously with the help of polarization data. All the group theoretically predicted Raman active fundamentals have been observed.

The dielec. behavior of $KClO_4$ was studied at low temp. to liq. He temp. and confirmed by DSC. The dielec. and thermal studies of $KClO_4$ at low-temp. indicates the presence of a low-temp. structure transition with considerable thermal hysteresis (.apprx.$20^0$). The intensity of the dielec. peak reduces considerably during successive thermal cycling. The dielec. anomaly might be due to a disorder in the orientation of the $ClO_4$ groups in the as-grown crystals. This disorder can contribute to the dielec. anomaly in the crystal. The disorder in the $ClO_4$ orientations in addn. to making a contribution directly, can also through the ionic polarization play a role in the anomaly. By thermal cycling this disorder possibly gets minimized and the structure acquires the spatial ordering corresponding to the space group Pnma and the dielec. anomaly slowly decreases

Raman spectra of the ferroelectric LiH3 (SeO3)2 and NaH3(SeO3)2 and the anti-ferroelectric KH3 (SeO3)2 have been recorded at room temperature using a He-Ne and also an Ar-ion laser source. The infrared absorption spectra of these crystals and their deuterated analogues have been recorded in the region 400–4000 cm−1 both below and above the Curie temperature. From an analysis of the spectrum in the region 400–900 cm−1 it is concluded that (i) in LiH3 (SeO3)2 the protons are ordered in an asymmetric double minimum potential with a low barrier and the spectrum can be interpreted in terms of HSeO3− and H2SeO3 vibrations, (ii) in NaH3 (SeO3)2 all three protons occupy a single minimum potential at room temperature and below the transition temperature the groups HSeO3− and H2SeO3 are present, (iii) the proton at the inversion centre in KH3(SeO3)2 is in a broad troughed potential well and the low temperature spectrum is more likely to be due to H3SeO3+ and SeO32− species. This deviation of the spectrum from that of the previous two crystals is attributed to the difference in H-bond scheme and hence the absence of any cooperative motion of protons in this crystal.

Dielectric observations on lithium hydrazinium sulphate have shown earlier that it is ferroelectric over a range of temperatures from below −15° C. to above 80° C. and a new type of hydrogen bond rearrangement which would allow the protons to migrate along the chain has also been suggested by others. The infrared spectrum of LiH z S in the form of mull and as single crystal sections parallel and perpendicular to the ‘C’ axis exhibit about 21 well-defined absorption maxima. The position and the width of the maxima agree with the known structure of the crystal according to which the hydrazine group exists in the form of the hydrazinium ion, NH2·NH3+ and the observed N+-H frequencies agree better with the new correlation curve given by R. S. Krishnan and K. Krishnan (1964). However it has been pointed out that from a comparative study of the new infrared spectra of hydrazonium sulphate and lithium ammonium sulphate that the absorption band at 969 cm.−1 is due to N-N stretching vibration and that the fairly intense band between 2050–2170 cm.−1 is due to the bending vibrations of the NH3+ group.

he infrared absorption spectra of glycine silver nitrate (GAgNO3) and glycine nitrate (GHNO3) show that the glycine group exists completely in the zwitter ion form in the former and in both forms in the latter. The spectrum of GAgNO3 at liquid air temperature did not reveal any striking change which can be attributed to a freezing of the rapid reorientation of the NH3+ group taking place at higher temperatures. The position of the COO− stretching frequencies indicate that this group is co-ordinated only weakly to the Ag+ ion. The summation frequencies reported by Schroeder, Wier and Lippincott (1962) for AgNO3 were not observed in the present study on GAgNO3. It shows however that ferroelectricity in GAgNO3 is in all probability due to the motion of the Ag+ ion in the oxygen co-ordination polyhedron and is not directly connected with the ordering of the hydrogen bonds below Curie point.

Raman spectrum of Tris-sarcosine Calcium chloride has been recorded both in the para and ferroelectric phases using \lambda = 2537 � of mercury as exciting radiation. The infrared absorption spectrum has been recorded both above and below the Curie temperature, in the region $400 to 4000 cm^{-1}$. From the observed splitting of the infrared bands it is concluded that the second order ferroelectric phase transition at 127K is due to a dynamical ordering of sarcosine molecules which exist in zwitteron configuration. Raman spectrum shows eight lattice lines which do not exhibit any anomaly as the crystal undergoes phase transition.

The emission spectrum of bismuth monobromide has been investigated and a vibrational analysis of the A→X system has been made. About 286 bands were recorded in the region λλ 4595–6063 and the isotope effect due to Br79 and Br81 was observed in about 87 bands. A value of 2·74 ev. for the dissociation energy of the excited state has been obtained and arguments have been given to show that the dissociation products in the excited state are Bi(4S3/2) and Br(2P3/2) and that those of the ground state are most probably Bi (4S3/2) and Br (2P1/2) atoms.

The observation of (A-X) system of BiF has been extended up to λ 5316 and twenty new bands belonging to this system have been recorded. The band heads could be represented by the following equation: {Mathematical expression} Seven other faint bands in the region λ 5316-5492 have also been reported, which, however, could not be classified. By our analysis of the present data and from known thermochemical data it has been deduced that the ground state dissociation energy is, in all probability, around 20000 cm.-1 (∼2·5 ev.) and that the dissociation products are the normal Bi and F atoms. The dissociation energy of the upper state and the correlation rules have been used to show that the dissociation products in the upper state are very likely to be Bi atom in the excited state2D3/2 and F atom in its ground state (2P3/2).

Teen the ehe dielec. consts. of thiourea powders of different particle sizes were detd. at 70-300 K, and the differences betwxptl.results and the theory of M. Marutake (1955) are discussed.