Solid State Sciences 4 (2002) 985–992 www.elsevier.com/locate/ssscie Synthesis, crystal structure and thermal behavior of Rb 3 B 7 O 12 , a new compound R.S. Bubnova a , S.V. Krivovichev b , I.P. Shakhverdova b , S.K. Filatov b,∗ , P.C. Burns c , M.G. Krzhizhanovskaya b , I.G. Polyakova a a Grebenshchikov Institute of Silicate Chemistry, Russian Academy of Sciences, ul. Odoevskogo 24/2, 199155 St. Petersburg, Russia b Department of Crystallography, St. Petersburg State University, Universitetskaya nab. 7/9, 199034 St. Petersburg, Russia c Department of Civil Engineering and Geological Sciences, 156 Fitzpatrick, University of Notre Dame, Notre Dame, IN 46556-0767, USA Received 5 November 2001; received in revised form 28 February 2002; accepted 28 March 2002 Abstract A new compound, Rb 3 B 7 O 12 , has been synthesized using solid state reaction and glass crystallization methods. It is triclinic, space group P-1, a = 6.603(1), b = 6.632(1), c = 30.085(6) Å, α = 91.183(4), β = 91.781(4), γ = 119.293(3) ◦ , V = 1147.5(5) Å 3 , Z = 4. The crystal structure of the compound has been solved by direct methods and refined to R 1 = 0.068 (wR = 0.142) on the basis of 1840 unique observed reflections (|F o | > 4σ |F o |). The structure is based upon a novel [B 7 O 12 ] layered anion. The layer is parallel to (001) and is composed of four types of rigid boron–oxygen groups: two [B 5 O 8.5 ] 2- pentaborate groups, [B 3 O 5 ] - triborate groups and BO 4 single tetrahedra. There are six symmetrically independent Rb atoms in the structure, of which four are located in voids within the borate layer and two are between the layers. High-temperature X-ray powder diffraction studies showed that the new compound forms above 620 ◦ C as a result of the solid state reaction Rb 2 B 4 O 7 + α-RbB 3 O 5 ↔ Rb 3 B 7 O 12 . The compound melts at ∼ 700 ◦ C according to the peritectic reaction Rb 3 B 7 O 12 ↔ β - RbB 3 O 5 + liquid. Thermal expansion of the structure of Rb 3 B 7 O 12 is highly anisotropic (α 11 = 34, α 22 = 9, α 33 = 2 × 10 -6 ◦ C -1 ). The thermal expansion is maximal in the direction perpendicular to the plane of the layer (α 11 = 34 × 10 -6 ◦ C -1 ).  2002 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. 1. Introduction Framework borates of alkali metals can exhibit unusual properties such as nonlinear optical behavior [1–6] and piezoelectricity [7]. Layered structures have long been important for ion-exchange reactions and, more recently, for the preparation of nanocomposites [8]. In the case of alkali metal borates, layered structures are relatively rare and have been reported for β -NaB 3 O 5 [9], Na 2 B 4 O 7 [10], α-CsB 5 O 8 [11], CaNaB 5 O 9 [12], SrKB 5 O 9 [13] and Cs 2 M 2 B 10 O 17 (M = Na, K) [8]. The discovery of promising new materials has prompted considerable interest in the generation of related compounds. We report here a new layered compound, Rb 3 B 7 O 12 , that was discovered during our studies of the Rb 2 O–B 2 O 3 system [14]. Compounds with stoichiometry 2M 2 O : 5B 2 O 3 (28.7 mol% M 2 O) have been reported in most phase diagrams * Correspondence and reprints. E-mail address: filatov@mail.ru (S.K. Filatov). of alkaline borate systems studied by thermal analyzes and X-ray powder diffraction. However, their crystal structures are poorly understood. Frequently, crystal structure analyzes demonstrate that these phases have a M 2 O : B 2 O 3 ratio of 3 : 7 (30 mol% M 2 O) instead of 2 : 5 (28.7 mol% M 2 O). For example, in the Li 2 O–B 2 O 3 system, structure studies of a compound reported to have a M 2 O : B 2 O 3 ratio of 2 : 5 showed it has a 3 : 7 ratio [15], although it is still consid- ered by some authors as a 2 : 5 compound [16]. Two lay- ered compounds, Cs 2 M 2 B 10 O 17 (M = Na, K) with 2 : 5 sto- ichiometry, have recently been reported [8]. In the Rb 2 O– B 2 O 3 system, the 2Rb 2 O·5B 2 O 3 compound was first found by Toledano [17,18] and its powder diffraction pattern has been deposited in the powder diffraction database (PDF, No. 19-1084). However, later, existence of this phase was not confirmed [19] and has been questioned until recent crystal- structure determinations of Rb 2 B 4 O 7 [20], α-RbB 3 O 5 [21] and β -RbB 3 O 5 [22], Rb borates with adjacent composi- tions. A topological polymorphic transition has been found in the case of RbB 3 O 5 [23,24]. Recently, we were able to obtain crystals of the compound with 3 : 7 stoichiometry, 1293-2558/02/$ – see front matter  2002 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. PII:S1293-2558(02)01341-9