1 Rus sian Re search Cen tre "Kurchatov In sti tute", Mos cow, Rus sia 2 In sti tute of Ge ol ogy of Ore De po si tions of RAS, Mos cow, Rus sia 3 Mos cow NPO "Ra don", Mos cow, Rus sia Sci en tific pa per Two pyrochlore ce ramic sam ples were stud ied in this work. The X-ray dif frac tion and the scanning elec tron mi cros copy showed that the ce ram ics with the cal cu lated com po si tion CaThSn 2 O 7 was formed by the dom i nat ing pyrochlore phase with the traces of thoria nite and he ma

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... te, while the CaThZr 2 O 7 ce ram ics -by the dom i nat ing pyrochlore phase with the mi nor ad mix tures of thoria nite and perovskite. The real com po si tions of pyrochlore phases de termined by the scaning elec tron mi cros copy are Ca 0.88 Th 0.92 Sn 2 O 6.72 and Ca 0.84 Th 0.80 Zr 2 O 6.44 . On the ba sis of the X-ray pho to elec tron spec tral pa ram e ters of the outer and core elec trons in the bind ing en ergy range of 0-1250 eV it was found that tin, zir co nium and tho rium in pyrochlore are at least 93%-94% tetravalent. Sn-O and Zr-O in ter atomic dis tances in BO 6 -octa he drons in the pyrochlore were found to be 0.210 nm and 0.220 nm, re spec tively, and these oc ta he drons are pos si ble to be tetragonaly dis torted. Key words: X-ray pho to elec tron spec tros copy, outer and in ner va lence mo lec u lar orbitals, pyrochlore, ra dio ac tive waste dis posal A. Yu. Teterin, et al.: The XPS Study of Pyrochlore Ma trixes for the Ra dio ac tive .