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Complex anionic isomorphism in new non-centrosymmetric  fluoride borates

Tatyana Bekker 1,  Sergey Rashchenko ,  Vladimir Bakakin ,  Yurii Seryotkin ,  Pavel Fedorov 2,  Alexander Yelisseyev ,  Alexander E. Kokh 1

1. Institute of Geology and Mineralogy SB RAS (IGM), Koptyuga ave., 3, Novosibirsk 630090, Russian Federation
2. General Physics Institute, Vavilov Str. 38, Moscow 117942, Russian Federation

Abstract
Isomorphic substitution is a very powerful and flexible tool to control the properties of materials, stabilize their desired crystal structure and optimize the conditions of their synthesis.  This phenomenon exists in various forms, such as isovalent/heterovalent isomorphism as well as cationic/anionic and combined cationic-anionic isomorphism, that are well represented among natural and man-made materials.  Anionic, specifically heterovalent anionic isomorphism, with a change in the number of ions in the unit cell (e.g., heterovalent inclusion of oxygen in tysonite lattices in LnF3-Ln2O3 systems, where Ln = rare earth element [Fedorov P. P.  Russ. J. Inorg. Chem.  45 (Suppl. 3), S268-S291 (2000)]) is a much rarer phenomenon than the other types of isomorphism.

Inorganic borates are quite famous for their nonlinear optical properties and transparency in the UV range of the electromagnetic spectrum.  Fluoride borates stand out amongst other borates, as they possess much broader areas of transparency with the cut-off edges shifted further in the aforementioned UV range [Wu, B. C., Tang, D. I., Ye, N.and Chen, C. T.  Opt. Mater.  5, 105–109 (1996)]. 

We prepared a new Ba4-xSr3+x(BO3)4-yF2+3y phase (space group P63mc) and grew its single crystals in BaSrBO3F–NaF system [Rashchenko S.V., Bekker T.B., Bakakin V.V., Seryotkin Yu.V., Shevchenko V.S., Kokh A.E., Stonoga S.Yu.  Cryst. Growth & Design. 2012, 12, 2955-2960].  The distinguishing feature of its structure is the simultaneous Ba2+ ↔ Sr2+ cationic and (BO3)3– ↔ 3F– anionic isomorphism.  No stoichiometric Ba3Sr4(BO3)3F5 compound has been found: its existence is not supported by experimental data.  Another new Ba7(BO3)4–yF2+3y solid solution with similar (BO3)3– ↔3F– anionic isomorphism has been found in BaB2O4–BaF2–BaO ternary system (0.21<y<0.65, Ba7(BO3)3.35F3.95 to Ba7(BO3)3.79F2.63 area of homogeneity, for y = 0.49, Ba7(BO3)3.51F3.47, a=11.18241(11) Å, c=7.23720(8) Å, space group P63) [Bekker T.B., Rashchenko S.V., Bakakin V.V., Seryotkin Yu.V., Fedorov P.P., Kokh A.E., Stonoga S.Yu.  CrystEngComm.  2012, 14, 6910-6915]. 

Both Ba4-xSr3+x(BO3)4-yF2+3y and Ba7(BO3)4–yF2+3y structures present  a previously-unknown mechanism of (BO3)3– ↔ 3F– heterovalent anionic substitution in fluoride borates via [(BO3)F]4– ↔ 4F– replacement, where the fourth fluorine atom in these fragments play a crucial role in the replacement of three fluoride ions by the orthoborate ion. 

Variation of Ba4-xSr3+x(BO3)4-yF2+3y and Ba7(BO3)4–yF2+3y compositions allows controlling their optical properties.

 

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Presentation: Oral at 17th International Conference on Crystal Growth and Epitaxy - ICCGE-17, Topical Session 7, by Tatyana Bekker
See On-line Journal of 17th International Conference on Crystal Growth and Epitaxy - ICCGE-17

Submitted: 2013-03-28 07:05
Revised:   2013-03-28 10:39