On the Possibility of Observing the Incommensurately Modulated Modification of Li 2 B 4 O 7
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On the Possibility of Observing the Incommensurately Modulated Modification of Li2B4O7 S. N. Volkova, *, S. A. Petrovab, L. I. Isaenkoc, and R. S. Bubnovaa aGrebenshchikov
Institute of Silicate Chemistry, Russian Academy of Sciences, St. Petersburg, 199034 Russia of Metallurgy, Ural Branch, Russian Academy of Sciences, Yekaterinburg, 620016 Russia c Sobolev Institute of Geology and Mineralogy, Siberian Branch, Russian Academy of Sciences, Novosibirsk, 630090 Russia *e-mail: [email protected] bInstitute
Received December 12, 2019; revised February 3, 2020; accepted April 3, 2020
Abstract—Single crystal studies of the Li2B4O7 borate in a wide temperature range are performed. The studies confirm the smoothed nature of the temperature dependences of the c cell parameter and do not reveal any modulated phases at low temperature. Keywords: crystal structure, thermal expansion, X-ray diffraction DOI: 10.1134/S1087659620040136
INTRODUCTION Li2B4O7 is a well-known optical material that is applied as a Glan–Thompson polarizer for deep ultraviolet radiation, as an electro-optical modulator, and in nonlinear optics [1, 2]. The information on thermal expansion and on the presence of a low-temperature incommensurately modulated modification of lithium tetraborate Li2B4O7 is contradictory. There is evidence of the presence of a low-temperature incommensurately modulated modification of Li2B4O7, which can be stabilized by thermal cycling in the temperature range of 100–300 K [3–5]. There is evidence of a stepwise thermal expansion of this phase in the range of 100–300 K, which was observed by the dilatometry [6] and also by single-crystal X-ray diffraction [7]. Similar thermal expansion was not previously observed in other compounds. Anomalies in the temperature dependence of the dielectric constant were observed in [8]. In [9], the authors were not able to observe any anomalies by NMR spectroscopy in the thermal behavior, which called into question the existence of polymorphic transitions in this borate. Later in [10, 11] in the study of Li2B4O7 using powder neutron diffraction in the temperature ranges 3.4–268 K and 293–1203 K, respectively, it was also not possible to observe any anomalies in the thermal behavior. There are still many questions regarding the nature of the thermal expansion of this borate at low temperatures. To resolve them, single crystal studies were carried out at temperatures of 100 to 300 K with a step of 1 K.
EXPERIMENTAL The single Li2B4O7 crystal ~1 × 2 × 0.4 cm in size was grown from a melt by the Czochralski method. In total, three samples of Li2B4O7 crystals were studied. Two 10 × 4 × 0.5-mm plates were cut out from a crystal perpendicular to the c axis. These were studied by X-ray diffraction in the temperature range 100–300 K in increments of 1 K. The first plate was examined on a Bruker D8 ADVANCE diffractometer (CuKα) equipped with an Anton Paar TTK450 thermal attachment. The position of 0.0.12 reflection in the range of angles 2θ of 123° to 132° was measured, after expos
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