Structural Instability in Single-Crystal Rare-Earth ... - ACS Publications

Subscriber access provided by READING UNIV

Article 3

4

Structural instability in single-crystal rare-earth scandium borates RESc(BO3) Galina M. Kuzmicheva, Irina A. Kaurova, Victor B Rybakov, Vadim V. Podbel'skiy, and Nikolay K. Chuykin Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b01534 • Publication Date (Web): 12 Feb 2018 Downloaded from http://pubs.acs.org on February 13, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Crystal Growth & Design is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Crystal Growth & Design

Structural instability in single-crystal rareearth scandium borates RESc3(BO3)4 Galina M. Kuz’michevaa, Irina A. Kaurovaa,*, Victor B. Rybakovb, Vadim V. Podbel’skyс, Nikolay K. Chuykinс a

Moscow Technological University, Institute of Fine Chemical Technologies, Vernadskogo pr. 86, Moscow, 119571 Russia b

с

Lomonosov State University, Vorobyovy Gory, Moscow 119992, Russia

National Research University «Higher School of Economics», Myasnitskaya str. 20, Moscow, 101000 Russia

KEYWORDS: huntite family; optical material; crystal growth; rare-earth cations; X-ray diffraction (XRD); structure; point defects

ACS Paragon Plus Environment

1

Crystal Growth & Design 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 35

Corresponding Author * Dr. Irina A. Kaurova Department of Materials Science and Technology of Functional Materials and Structures, Moscow Technological University, Institute of Fine Chemical Technologies, 86 Vernadskogo pr., Moscow 119571, Russia Tel.: +7 495 246 05 55 (434), Fax.: +7 495 434 87 11 E-mail address: [email protected]


2

Page 3 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ABSTRACT

An influence of the type of rare-earth (RE) cation and composition of initial charge on the symmetry, structural features, and real composition of the single-crystal huntite-family rare-earth scandium borates RESc3(BO3)4 (RE = Ce, Pr, Nd), grown by the Czochralski method, has been studied by single crystal and powder X-ray diffraction. A crystallization of scandium borates in the space groups С2/c (RE = Ce) and P321 (RE = Pr, Nd) has been found. Disordering in the structures with the space group P321, which has been first determined for the huntite-family compounds, is due to the RE and Sc redistribution over two trigonal-prismatic sites to maintain the stability of crystal structure. The crystals grown from the initial charges NdSc3(BO3)4 and Nd1.25Sc2.75(BO3)4 are characterized by the greatest disordering and they are isotypic, rather than isostructural, to the crystals obtained from the charges Pr1.1Sc2.9(BO3)4 and Pr1.25Sc2.75(BO3)4. A change in the unit cell parameters and interatomic distances depending on the RE radius and composition of the initial charge is found and explained. Analysis of our and literary data for RESc3(BO3)4 with the RE = La, Ce, Pr, Nd, Sm, Eu allowed to reveal a morphotropic series for scandium borates based on the size factor (the RE3+ radius), suggest methods for evaluating the specific crystal symmetry depending on the type of RE cation, and propose factors affecting its realization. The comparison of huntite-family REМ3(BO3)4 (М = Al, Ga, Sc) single crystals allowed to establish basic differences between their crystal structures, mainly due to the different sizes of RE and M ions.


3


Page 4 of 35

1. Introduction Rare-earth scandium borate crystals and solid solutions with the corresponding general compositions RESc3(BO3)4 and (RE,RE’)Sc3(BO3)4 (RE, RE’ are rare earth elements) belonging to the huntite family (huntite CaMg3(CO3)4, the space group R32) are related to promising optical materials used in photonics, in particular, to create diode-pumped high-efficiency compact lasers covering various spectral regions [1-3]. These media are characterized by a high absorption efficiency for the pump radiation, which greatly contributes to the achieving a maximum pump power density in an active element [2]. Physical and chemical properties of rare-earth scandium borates, described in detail [1-3], are due to their crystal structure. These crystals demonstrate an anomalously low luminescence concentration quenching, which is caused by a large distance between the nearest RE ions (~ 6 Å). Due to the non-centrosymmetric structure, scandium borate crystals possess nonlinear optical properties, what ensure a self-doubling of laser generation frequency [2]. RESc3(BO3) crystals may have different symmetry depending on the type of RE cation. LaSc3(BO3)4, LSB. Three modifications are known: a high-temperature phase αLaSc3(BO3)4 with the space group C2/c, a medium temperature phase β-LaSc3(BO3)4 with the space group R32 (huntite structure), and a low-temperature phase γ-LaSc3(BO3)4 with the space group Cc. The temperature of the phase transition from γ-LaSc3(BO3)4 to β-LaSc3(BO3)4 is 1050 °C, whereas from β-LaSc3(BO3)4 to α-LaSc3(BO3)4 is 1200 °C [4]. All the modifications were grown by flux and Czochralski methods from stoichiometric melts or by solid state synthesis. An X-ray diffraction study of powdered LSB single crystals obtained by a solid state synthesis and heat-treated at temperatures from 1000 to 1350 °C showed that the α and γ-phases of LaSc3(BO3)4 are identical and belong to the sp. gr. С2/с [5]. In addition, Fedorova et al. [5], Li et


4

Page 5 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


al. [6], and Ye et al. [7] failed to obtain a stable β-phase by a solid state reaction, suggesting that this phase is metastable or stable in a narrow temperature range. According to [8-10], in the structures of trigonal R32 and monoclinic C2/c modifications, the La atom is located at the center of a distorted trigonal prism with similar interatomic distances (CNLa = 6, CN is a coordination number) and with three different ones (CNLa = 2 + 2 + 2), respectively, whereas in the monoclinic Сс structure, the La atom occupies a distorted octahedron with all different La-O distances (according to [4]). In the LSB crystal structures with the space groups Сс, С2/с and R32, the Sc atoms occupy three (Sc1, Sc2, Sc3 - all the interatomic distances are different), two (CNSc1 = 2 + 2 + 2; Sc2 - all the distances are different) and one (CNSc = 2 + 2 + 2) crystallographic sites, respectively, forming distorted octahedra. The B1 and B2 atoms are surrounded by the O atoms forming regular and isosceles (the space group R32) or scalene (the space group С2/с) triangles. In turn, in the structure with the space group Сс, the B atoms occupy four different scalene triangles [4]. The LSB crystal structures were solved within the framework of the space groups Cc [4], R32 [8, 9], and C2/c [10] without any refinement of site occupancies, i.e. the real compositions were considered to be stoichiometric. Fig. 1 shows the combination of coordination polyhedra in all the structures. A comparison of the combination of coordination polyhedra in the structures with the space groups Сс and R32 shows that a transition from the low-symmetry γ- to the high-symmetry β-modification is logically accompanied with a decrease in the number of different structural fragments for cations: one (Sc) and two (B1 and B2) structural fragments (the space group R32) are formed from three Sc (Sc1, Sc2, Sc3) and four B (B1, B2, B3, B4) ones (the space group Cc), respectively.


5


Page 6 of 35

СeSc3(BO3)4, CSB. According to [11, 12], the CSB can possess trigonal (the space group R32) or monoclinic (the space group C2/c) symmetry depending on a synthesis method. Coordinates of atoms, thermal parameters, interatomic distances, and bond angles in the CSB crystals obtained by the flux method were refined within the framework of the space group R32 [11], however, the precise real composition was not determined. The CSB crystals grown by the Czochralski technique were found to be crystallized in the space group C2/c, but no any refinement of the crystal structure was performed. PrSc3(BO3)4, PSB. When refining the crystal structure of PSB crystal grown by the Czochralski method from the initial charge with composition Pr1.1Sc2.9(BO3)4, the extinction laws for an overwhelming number of diffraction reflections witness crystallization of this compound in the space group C2/c or Сс. However, a small number of additional reflections with I ≥ 3σ(I) was revealed, which are characteristic for the space group C2/m, С2 or Сm (h + k = 2n for hkl; h = 2n, l = 2n for h0l; h = 2n for h00) [2, 12]. For these crystals, a non-synchronous second harmonic generation was observed, that indicates a noncentrosymmetric structure, which, most likely, crystallizes in the space group C2 as a subgroup of the C2/c known for huntite-family compounds. Due to a small number of additional reflections (~5%), coordinates of atoms, thermal parameters, interatomic distances, and bond angles of PSB crystal structure were refined in the space group C2/c [12]. In addition, the structure of the crystals with the charge composition PrSc3(BO3)4 was solved in the space group R32 [13, 14]. In all the cases, site occupancies were not refined [2, 12 - 14]. NdSc3(BO3)4, NSB. The structure of NSB crystals grown from the initial charge with the composition NdSc3(BO3)4 was refined within the framework of the space group R32 [13, 14]. The unit cell parameters of crystal with the initial charge composition Nd1.25Sc2.75(BO3)4,


6

Page 7 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


determined by the auto-indexing of 21 reflections ( h0h 0 , 000l) in the range of interplanar distances d = 2.01 - 8.14 Å, correspond to the primitive trigonal cell (a = 9.74, c = 15.83 Å) with the double c parameter compared with the huntite structure (the space group R32) [15]. In the range of d = 3.96 - 4.00 Å, several broad reflections with a width of 1.23 - 1.4° were detected (the remaining reflections of approximately the same intensity had a width of 1.05°). As a result, a primitive trigonal cell with the double parameters a = 19.526(3) and c = 15.838(2) Å was found. The presence of such diffraction reflections indicates a disordering in the crystal structure. The structure of the crystal with the charge composition Nd1.25Sc2.75(BO3)4 was refined in the space groups R32, P321, and P3 (the P321 and P3 are the most probable) with the unit cell parameters a = 9.763(3), c = 7.919(2) Å [15]. In all the cases, anomalous (negative) thermal parameters for the O and B atoms were observed, that indicates problems in the X-ray diffraction analysis of NSB. The Nd and Sc site occupancies were only refined by the Rietveld method for powdered single crystals with the charge composition Nd1.25Sc2.75(BO3)4 (the space group R32) [15]. As a result, the crystal composition was found to be stoichiometric, i.e. NdSc3(BO3)4. RESc3(BO3)4 (RE = Sm, Eu). The structures of Czochralski-grown crystals with the charge compositions RESc3(BO3)4 with RE = Sm, Eu were refined in the space group R32 [14]. However, the precise real composition was not determined, i.e. any refinement of site occupancies was not performed. Thereby, there are only several works devoted to the investigation of RESc3(BO3)4 crystal structure, their results being incomplete, contradictory, and, in some cases, questionable. Up to this time, there is no unified opinion on a symmetry and structure of these compounds. In addition, any correlations between structure, precise real crystal composition (in the majority of cases, it was not determined or determined for powdered crystals only), and synthesis conditions,


7


Page 8 of 35

in particular, initial charge composition, were not revealed. It is known that a real composition of compounds, taking into account a deficiency of all crystallographic sites, may differ from a composition of initial charge and, hence, correlations between composition and functional properties will be changed [16, 17]. A correct determination of symmetry and structure of huntite-family rare-earth scandium borates becomes very important, since a possible structural transition from one space group to another can be accompanied with a loss (or acquisition) of the center of symmetry and results in a loss (or acquisition) of nonlinear optical properties in a laser crystal. It was found that the introduction of 5 at.% Nd3+ ions (the corresponding concentration was determined to be 2.3 × 1020 cm-3) into LaSc3(BO3)4 results in a transition from the space group C2/c (LSB) to C2 (LSB:Nd), i.e. a transition from a centrosymmetric to a noncentrosymmetric structure; it is accompanied with a fundamental change in the properties [18]. All these structural aspects are not given much attention in modern materials science. The aim of this work is to establish dynamics and causes for structural transitions in the huntite-family RESc3(BO3)4 compounds depending on the type of rare-earth (RE) cation. 2. Materials and Methods The RESc3(BO3)4 (RE = Ce, Pr, Nd) single crystals were grown by the Czochralski technique in iridium crucibles of 40 mm in diameter in a "Kristall-3" unit. The pulling rate was 1-2 mm/h and the seed was rotated at 8-10 rpm. The seed was oriented so that its optical axis coincided with the pulling axis (within a few degrees). The growth time was 60-70 h. The average diameter of the grown crystals was 15-20 mm and the average length was 10-15 cm. The conditions used to grow crystals are described in detail in [19]. The objects of investigation are given in Table 1.


8

Page 9 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The X-ray powder diffraction (XRPD) data for samples ground to a powder were collected in a reflection mode at room temperature on a HZG-4 (flat graphite monochromator) X-ray powder diffractometer using CuKα radiation and a diffracted beam (step-scan mode; the count time was 15 s and the step size was 0.02°) in the 2θ angle range of 10–80°. The qualitative phase analysis of the samples, which was performed with the PCPDFWIN automatic search software for reading the PDF-2 database, showed that all the samples were single phase. The unit cell parameters were refined by the FullProf-2007 program [20]. The X-ray diffraction (XRD) analysis of microcrystals ~ 0.1 × 0.1 × 0.1 mm3 in size was carried out on a Enraf-Nonius CAD-4 single-crystal diffractometer at room temperature (MoKα or AgKα, graphite monochromator, ω/2θ scan mode) (Table S1, ESI). To reduce an error associated with the absorption, the XRD data were collected over the entire Ewald sphere. The preliminary XRD data processing was carried out using the WinGX pack [21] with a correction for absorption (MULTISCAN). The crystal structures of the compounds are solved by the Paterson method and direct (statistical) methods [22]. The atomic coordinates, anisotropic displacements parameters for all the atoms, and occupancies of cation and oxygen sites were refined using the SHELXL2013 software package [22], taking into account the atomic scattering curves for neutral atoms. The structural parameters were refined in several steps. Initially, the coordinates of "heavy" (RE, Sc) and "light" (O and B) atoms were refined with fixed thermal parameters. Then the refinement of the thermal parameters in isotropic and anisotropic approximations was performed in the same order with fixed positional parameters. Finally, the occupancies of RE, Sc, and O sites were refined step by step. The electroneutrality of the system, the number and grade of atom location, crystallochemically acceptable interatomic distances, real thermal parameters, and the lowest


9


Page 10 of 35

values of the R-factors served as criteria for the accuracy and correctness of the refinement performed. Crystallographic data have been deposited with the Cambridge Crystallographic Data Centre, the CCDC codes 1575235-1575236. The results of the refinement of atomic coordinates, thermal parameters, site occupancies, and interatomic distances in the space group P321 (RE = Pr, Nd) and C2/c (RE = Ce) are given in Tables S1-S4, ESI. The “Program for investigation the dynamics of changes in the structural parameters of compounds with different symmetry" has been developed to visualize the general crystal structure, combination of coordination polyhedra, and individual polyhedra, to calculate structural parameters (interatomic distances, bond and distortion angles), to build correlations between structural parameters and a size of ions for the huntite-family compounds having different symmetry. The basic characteristics: CPU, Intel Core i5 6600k; RAM, at least 8 GB; Programming language, C#; OS, Windows 10 with the Microsoft .NET Framework 4.0 or higher; Size, 17 756 KB. 3. Results and Discussion According to [12, 23], the compounds with the composition RESc3(BO3)4 form a morphotropic series, which can be conventionally divided into huntite- (RE = La - Eu with rREVI = 1.02 - 0.95 Å [24], r is a cationic radius) and non-huntite family compounds, e.g. the compound with the RE = Tb (rTbVI = 0.92 Å) crystallizes in the superstructure to the ScBO3 (the _

space group R 3 ). Hence, the reason for the formation of a morphotropic series is the size factor. In the huntite-type structure, the RE atoms are located in trigonal prisms, except for the lowtemperature modification with the space group Cc, in which the coordination polyhedron for RE is a distorted octahedron [4]. At the same time, two crystallochemically-different Tb atoms (possibly together with the Sc atoms) occupy the centers of distorted octahedra.


10

Page 11 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


It should be noted that the polyhedra derived from the regular trigonal prism (Fig. 2a) and regular octahedron (Fig. 2c) differ in a rotation angle between the upper and lower triangular faces (ϕ): ϕ = 0° for a trigonal prism and ϕ = 60° for an octahedron. It can be conditionally assumed that the ranges 0° < ϕ

Structural Instability in Single-Crystal Rare-Earth ... - ACS Publications

Recommend Documents