ISSN 1063-7745, Crystallography Reports, 2013, Vol. 58, No. 6, pp. 835–841. © Pleiades Publishing, Inc., 2013. Original Russian Text © S.M. Aksenov, V.A. Rassulov, R.K. Rastsvetaeva, V.K. Taroev, 2013, published in Kristallografiya, 2013, Vol. 58, No. 6, pp. 836–842. 835 INTRODUCTION Currently, the interest in alkaline microporous rare earth silicates is caused by their photoluminescence and ion-exchange properties. This study continues the systematic investigations of the hydrothermal synthe- sis and structure of alkaline rare earth silicates to establish the conditions for incorporating lanthanides into solid phases. Previously, europium-containing compounds were synthesized and studied. Eu 3+ -containing phases were synthesized in Cu containers under oxidative condi- tions corresponding to the Cu–Cu 2 O buffer associa- tion. Their structures have layered [Si 6 O 15 ] and ribbon [Si 6 O 17 ] and [Si 8 O 21 ] motifs [1]. The europium-con- taining phase, synthesized under reducing conditions (corresponding to the Ni–NiO buffer pair), is charac- terized by a framework structure composed of silicon– oxygen nanotubulenes [2]. In this study, we obtained two novel phases, K 4 Ce 2 [Al 2 Si 8 O 24 ] and K 4 Gd 2 [Al 2 Si 8 O 24 ], by hydrothermal synthesis and studied their crystal structures and luminescence properties. SYNTHESIS AND ANALYSIS OF SAMPLES The novel compounds were synthesized in stainless steel autoclaves, equipped with nickel containers (sealed by argon-arc welding before experiments), under a total pressure of 100 MPa in aqueous alkaline solutions with a KOH concentration of 15.25 wt %. The use of nickel containers for synthesizing Ce- and Gd-containing silicate phases makes it possible to maintain the oxygen potential at a constant level, cor- responding to the Ni–NiO buffer association; this condition is especially important for experiments with cerium (element of variable valence). The temperature (500°С) in the experiments, which lasted for 35 days, was maintained with an error of ±5°С, with allowance for the multiday temperature drift. SiO 2 , Al 2 O 3 , CeO 2 , and Gd 2 O 3 oxides of special- purity grade with a molar ratio SiO 2 /Al 2 O 3 = 11.21 were used as the initial charge. The temperature drop ΔТ between the lower and upper parts of autoclaves was varied from 8 to 10°С. After exposure of auto- claves under stationary conditions, they were cooled in cold continuous-flow water. Synthesized colorless transparent crystals have an elongated prismatic shape (Fig. 1). With a low oxygen volatility, corresponding to the Ni–NiO buffer, the additional phase observed in the experiments is Ni mica, which forms flaky clusters (fringes), shorter than 10 μm, around Ce-containing crystals. Some crystals contain inclusions of potassium feldspar. Note that the experiments on synthesis of Eu-containing phases under reducing conditions in STRUCTURE OF INORGANIC COMPOUNDS Crystal Structures and Luminescence Properties of Novel Compounds K 4 M 2 [Al 2 Si 8 O 24 ] (M = Ce, Gd) S. M. Aksenov a , V. A. Rassulov b , R. K. Rastsvetaeva a , and V. K. Taroev c a Shubnikov Institute of Crystallography, Russian Academy of Sciences, Leninskii pr. 59, Moscow, 119333 Russia e-mail: aks.crys@gmail.com b Fedorovsky All-Russian Research Institute of Mineral Resources, Russian Academy of Sciences, Moscow, 119017 Russia c Institute of Geochemistry, Siberian Branch, Russian Academy of Sciences, ul. Favorskogo 1a, Irkutsk, 664033 Russia Received February 6, 2013 Abstract—Two novel potassium rare earth silicates, obtained by hydrothermal synthesis, have been investi- gated by X-ray diffraction and described by the general formula K 4 M 2 [Al 2 Si 8 O 24 ] (M = Ce, Gd). The param- eters of the monoclinic K 4 Ce 2 [Al 2 Si 8 O 24 ] and K 4 Gd 2 [Al 2 Si 8 O 24 ] cells are, respectively, as follows: a = 26.867(1), 26.6520(2) Å; b = 7.4150(2), 7.2854(1) Å; c = 14.910(1), 14.8182(1) Å; β = 123.52(1)°, 123.46(1)°; and sp. gr. P2 1 /n. The structures are solved by the charge flipping method and refined in the anisotropic approximation of thermal vibrations for atoms to R = 5.2 and 2.5%, respectively. The compounds under study are crystallized into a new structural type, which is based on two-level [Al 2 Si 8 O 24 ] layers, com- bined into a three-dimensional framework by columns of edge-sharing seven-vertex REE polyhedra. Potas- sium atoms are located in the framework channels. The spectral luminescence characteristics are determined. The luminescence bands are typical of Gd 3+ and Ce 3+ ions. Upon excitation by light with λ exc = 246 nm, a band due to the 2 D → 8 F 5/2 transition with λ max = 430 nm is observed in the spectrum of K 4 Ce 2 [Al 2 Si 8 O 24 ] and a band related to the 6 P 7/2 → 8 S 7/2 transition with λ max = 311 nm is observed in the spectrum of K 4 Gd 2 [Al 2 Si 8 O 24 ]. DOI: 10.1134/S1063774513060023 On the 70th Anniversary of the Shubnikov Institute of Crystallography of the Russian Academy of Sciences