a), N. A. KORYNEVSRII (a), and I. V. STASYUR (b) I n the cluster approximation, taking into account long-range correlations, the distribution functions of deuterated ferroelectric compounds of the orthophosphate type are calculated. The behaviour of the single and pair function near the phase transition point for both the KD,PO, and ND,D,PO, type structures is analysed. It is shown that expressions for the longitudinal and transversal components of the dielectric susceptibility tensor are in a qualitative coincidence with the experiment in the phase tramition region. Phase transition and some thermodynamical properties of deuterated orthophosphates are studied on the basis of the results obtained. It is shown that the long-range interaction is necessary to form entipolarization states. B KJIaCTepHOM n p n 6 n u m e~~n C Y W T O M HaJIbHOHefiCTByIoIU€ ¶X HOppeJIHlUifi IIOJIyqeHJd @J'HKU;EIH paClIpeAeJIeHHH nefiTepHpOBaHHblX CerHeTOaHTkiBHbIX COennHeHHfi TMna OPTO-@oc@aToB. JCaH aHanna noBeneHurr yHapHoti ~3 napmx I$YHKU;H~~ AJIH CTPYKTYP Tnna KD,PO, n Tnna ND,D,PO,. HonyseHHue BbxpameHm ~J I H npononbHMx H nonepewmx HOMl l OHeHT TeH3Opa aM3JIeKTpH'IeCKOfi BOCnPUHMWi BOCTU AaIoT B o6nac~u (pa30BOrO nepe-XOAa KageCTBeHHOe COrJIaCHe C 3KCllePHMeHTaJIbHbIMH AaHHbIMH. Ha OCHOBe IIOJIyWHHhIX AefiTepupOBaHHbIX OpTO@OC@aTOB. npH 3TOM IlOHa3aHa pt?IlIaIoIUaH pOJIb AaJIbHOAeftCT-BJ'IOWefi COCTaBJIHIoIUefi BaaAMOAe~CTBHR B YCTaHOBJIeHEiH COCTOHHHH aHTHIlOJIRPW3aU;HH. p e a y n b~a~o~ naysew Qaaos~fi nepexoa n HeKoTopme tepMonwHaMnnecme cBoficTsa 4.

The critical part of the free energy of ferroelectric clusters with dipole-dipole interactions between the particles is obtained. The calculation of physically different contributions to the total free energy is performed. The recursion relations for the coefficients of block Hamiltonians are solved in several approximations. The suitability is examined of each approximation to the appropriate type of critical behaviour of the system. The values of critical exponents of correlation length and heat capacity are calculated.Der kritische Anteil der Freien Energie von ferroelektrischen Clustern mit Dipol-Dipol-Wechselwirkungen zwischen den Teilchen wird erhalten. Dam wird eine Berechnung der physikalisch unterschiedlichen Beitrage zur gesamten Freien Energie durchgefiihrt. Die Rekursionsbeziehungen fur die Koeffizienten der Block-Hamiltonoperatoren werden in verschiedenen Naherungen gelost. Die Eignung jeder Naherung fur den passenden Typ des kritischen Verhaltens des Systems wird untersucht. Die Werte der kritischen Exponenten der Korrelationslange und Warmekapazitat werden berechnet.

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