X-ray diffraction and Mössbauer spectroscopy study of oxoborate azoproite (Mg,Fe²⁺)₂(Fe³⁺,Ti,Mg,Al)O₂(BO₃): an in situ temperature-dependent investigation (5 ≤ T ≤ 1650 K)

This work is devoted to an investigation of elemental composition, crystal structure and thermal expansion of natural oxoborate azoproite from the Tazheran massif (Siberia, Russia) in the temperature range 5–1650 K. Elemental composition was determined by energy-dispersive X-ray spectroscopy (EDX). Its empirical formula based on five oxygen atoms is (Mg_1.81Fe²⁺_0.19)_∑2.00(Fe³⁺_0.36Ti_0.26Mg_0.26Al_0.12)_∑1.00O₂(BO₃). Local environment, oxidation states and ratio of Fe atoms are determined using Mössbauer spectroscopy and compared with EDX and single-crystal X-ray diffraction (SCXRD) data. A refinement of the crystal structure from SCXRD data collected at 293 K was provided for the first time. The structure could be described both in terms of cation- and anion-centered polyhedra. It is composed of vertex- and edge-sharing metal–oxygen [MO₆]ⁿ⁻ octahedra that form extended zigzag chains along the a axis building up a framework with the [BO₃]³⁻ triangles located in its distorted trigonal channels. From the other point of view, there are double chains consisting of oxocentred [OM₄]ⁿ⁺ tetrahedra and [OM₅]ⁿ⁺ tetragonal pyramids forming six-membered rings with the triangles in its cavities. Four non-equivalent Mⁿ⁺ sites are occupied by cations as follows: M(1) (2a) and M(2) (2d) – Mg, M(3) (4g) – Mg and Fe²⁺, M(4) (4h) – Fe³⁺, Ti⁴⁺, Mg and Al³⁺. According to differential scanning calorimetry, low- and high-temperature powder X-ray diffraction (LT- and HT-XRD) data, Mössbauer spectroscopy and magnetometry data (5 ≤ T ≤ 1650 K), there are no phase transitions obtained in the temperature range investigated. However, some anomalies in temperature dependencies of unit-cell parameters caused by a partial Fe²⁺ → Fe³⁺ oxidation are found in the range 873–1173 K. Azoproite melts at a temperature higher than 1600 K. Eigenvalues of the thermal expansion tensor are calculated for the oxoborate and thermal expansion is described in comparison with its crystal structure.

Computing details

(I)

Crystal data

Al0.1BFe0.45Mg2.15O5Ti0.3	F(000) = 362
Mr = 185.3	Dx = 3.603 Mg m^−3
Orthorhombic, Pbam	Mo Kα radiation, λ = 0.71069 Å
Hall symbol: -P -2xab;-2yab;-2z	Cell parameters from 3926 reflections
a = 9.2460 (14) Å	θ = 2.8–35.5°
b = 12.2731 (18) Å	µ = 3.13 mm^−1
c = 3.0092 (5) Å	T = 293 K
V = 341.48 (9) Å^3	Prism, black
Z = 4	0.4 × 0.1 × 0.1 mm

Data collection

Bruker CCD diffractometer	715 reflections with I > 3σ(I)
Radiation source: X-ray tube	Rint = 0.040
Graphite monochromator	θmax = 35.8°, θmin = 2.8°
Absorption correction: multi-scan (Krause et al., 2015)	h = −15→14
Tmin = 0.820, Tmax = 0.860	k = −19→19
5341 measured reflections	l = −4→4
872 independent reflections

Refinement

Refinement on F	0 restraints
R[F^2 > 2σ(F^2)] = 0.050	24 constraints
wR(F^2) = 0.069	Weighting scheme based on measured s.u.'s w = 1/(σ^2(F) + 0.0001F^2)
S = 2.76	(Δ/σ)max = 0.037
872 reflections	Δρmax = 0.62 e Å^−3
59 parameters	Δρmin = −0.62 e Å^−3

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq	Occ. (<1)
Fe4	0.73794 (7)	0.38401 (6)	0.5	0.0090 (2)	0.4
Ti4	0.73794 (7)	0.38401 (6)	0.5	0.0090 (2)	0.3
Al4	0.73794 (7)	0.38401 (6)	0.5	0.0090 (2)	0.1
Mg2	0.5	0	0.5	0.0137 (5)
Mg3	0.00090 (12)	0.28056 (11)	0	0.0162 (4)	0.95
Fe3'	0.00090 (12)	0.28056 (11)	0	0.0162 (4)	0.05
Mg4	0.73794 (7)	0.38401 (6)	0.5	0.0090 (2)	0.2
Mg1	0	0	0	0.0030 (4)
O5	0.3495 (3)	0.2628 (2)	0.5	0.0093 (7)
O3	0.6248 (3)	0.1420 (2)	0.5	0.0097 (7)
O1	0.8503 (3)	0.0423 (2)	0.5	0.0090 (7)
O4	0.1089 (3)	0.1432 (2)	0	0.0101 (7)
O2	0.3832 (3)	0.0757 (2)	0	0.0109 (7)
B1	0.2735 (4)	0.3608 (3)	0.5	0.0082 (9)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Fe4	0.0087 (3)	0.0092 (3)	0.0091 (3)	0.0010 (2)	0	0
Ti4	0.0087 (3)	0.0092 (3)	0.0091 (3)	0.0010 (2)	0	0
Al4	0.0087 (3)	0.0092 (3)	0.0091 (3)	0.0010 (2)	0	0
Mg2	0.0124 (9)	0.0141 (9)	0.0146 (10)	0.0013 (6)	0	0
Mg3	0.0157 (7)	0.0155 (7)	0.0174 (7)	−0.0008 (4)	0	0
Fe3'	0.0157 (7)	0.0155 (7)	0.0174 (7)	−0.0008 (4)	0	0
Mg4	0.0087 (3)	0.0092 (3)	0.0091 (3)	0.0010 (2)	0	0
Mg1	0.0044 (8)	0.0020 (7)	0.0026 (7)	−0.0001 (4)	0	0
O5	0.0088 (11)	0.0077 (11)	0.0114 (12)	−0.0005 (8)	0	0
O3	0.0085 (11)	0.0098 (12)	0.0107 (12)	0.0009 (8)	0	0
O1	0.0083 (11)	0.0096 (12)	0.0090 (11)	−0.0002 (8)	0	0
O4	0.0094 (12)	0.0067 (11)	0.0141 (13)	0.0007 (8)	0	0
O2	0.0079 (11)	0.0103 (12)	0.0146 (12)	0.0005 (9)	0	0
B1	0.0070 (16)	0.0102 (17)	0.0072 (16)	0.0024 (12)	0	0

Geometric parameters (Å, º)

Fe4—O5^i	2.076 (3)	Mg3—O3^vii	2.117 (2)
Fe4—O1^ii	2.107 (3)	Mg3—O3^viii	2.117 (2)
Fe4—O4^iii	1.9491 (17)	Mg3—O4	1.959 (3)
Fe4—O4^i	1.9491 (17)	Mg3—O2^vii	2.073 (3)
Fe4—O2^iii	2.0766 (18)	Fe3'—O5^vii	2.1229 (19)
Fe4—O2^i	2.0766 (18)	Fe3'—O5^viii	2.1229 (19)
Ti4—O5^i	2.076 (3)	Fe3'—O3^vii	2.117 (2)
Ti4—O1^ii	2.107 (3)	Fe3'—O3^viii	2.117 (2)
Ti4—O4^iii	1.9491 (17)	Fe3'—O4	1.959 (3)
Ti4—O4^i	1.9491 (17)	Fe3'—O2^vii	2.073 (3)
Ti4—O2^iii	2.0766 (18)	Mg4—O5^i	2.076 (3)
Ti4—O2^i	2.0766 (18)	Mg4—O1^ii	2.107 (3)
Al4—O5^i	2.076 (3)	Mg4—O4^iii	1.9491 (17)
Al4—O1^ii	2.107 (3)	Mg4—O4^i	1.9491 (17)
Al4—O4^iii	1.9491 (17)	Mg4—O2^iii	2.0766 (18)
Al4—O4^i	1.9491 (17)	Mg4—O2^i	2.0766 (18)
Al4—O2^iii	2.0766 (18)	Mg1—O1^ix	2.1093 (17)
Al4—O2^i	2.0766 (18)	Mg1—O1^x	2.1093 (17)
Mg2—O3	2.090 (3)	Mg1—O1^xi	2.1093 (17)
Mg2—O3^iv	2.090 (3)	Mg1—O1^iv	2.1093 (17)
Mg2—O2	2.0720 (18)	Mg1—O4	2.026 (3)
Mg2—O2^v	2.0720 (18)	Mg1—O4^xii	2.026 (3)
Mg2—O2^iv	2.0720 (18)	B1—O5	1.393 (5)
Mg2—O2^vi	2.0720 (18)	B1—O3^viii	1.375 (5)
Mg3—O5^vii	2.1229 (19)	B1—O1^viii	1.385 (5)
Mg3—O5^viii	2.1229 (19)
O5^i—Fe4—O1^ii	172.99 (10)	O5^vii—Mg3—O3^viii	166.74 (12)
O5^i—Fe4—O4^iii	98.94 (9)	O5^vii—Mg3—O4	96.92 (10)
O5^i—Fe4—O4^i	98.94 (9)	O5^vii—Mg3—O2^vii	82.36 (9)
O5^i—Fe4—O2^iii	83.42 (9)	O5^viii—Mg3—O3^vii	166.74 (12)
O5^i—Fe4—O2^i	83.42 (9)	O5^viii—Mg3—O3^viii	88.03 (7)
O1^ii—Fe4—O4^iii	85.47 (9)	O5^viii—Mg3—O4	96.92 (10)
O1^ii—Fe4—O4^i	85.47 (9)	O5^viii—Mg3—O2^vii	82.36 (9)
O1^ii—Fe4—O2^iii	91.77 (9)	O3^vii—Mg3—O3^viii	90.61 (8)
O1^ii—Fe4—O2^i	91.77 (9)	O3^vii—Mg3—O4	96.35 (10)
O4^iii—Fe4—O4^i	101.06 (9)	O3^vii—Mg3—O2^vii	84.37 (10)
O4^iii—Fe4—O2^iii	82.96 (8)	O3^viii—Mg3—O4	96.35 (10)
O4^iii—Fe4—O2^i	174.90 (9)	O3^viii—Mg3—O2^vii	84.37 (10)
O4^i—Fe4—O2^iii	174.90 (9)	O4—Mg3—O2^vii	178.97 (12)
O4^i—Fe4—O2^i	82.96 (8)	O5^vii—Fe3'—O5^viii	90.27 (8)
O2^iii—Fe4—O2^i	92.86 (8)	O5^vii—Fe3'—O3^vii	88.03 (7)
O5^i—Ti4—O1^ii	172.99 (10)	O5^vii—Fe3'—O3^viii	166.74 (12)
O5^i—Ti4—O4^iii	98.94 (9)	O5^vii—Fe3'—O4	96.92 (10)
O5^i—Ti4—O4^i	98.94 (9)	O5^vii—Fe3'—O2^vii	82.36 (9)
O5^i—Ti4—O2^iii	83.42 (9)	O5^viii—Fe3'—O3^vii	166.74 (12)
O5^i—Ti4—O2^i	83.42 (9)	O5^viii—Fe3'—O3^viii	88.03 (7)
O1^ii—Ti4—O4^iii	85.47 (9)	O5^viii—Fe3'—O4	96.92 (10)
O1^ii—Ti4—O4^i	85.47 (9)	O5^viii—Fe3'—O2^vii	82.36 (9)
O1^ii—Ti4—O2^iii	91.77 (9)	O3^vii—Fe3'—O3^viii	90.61 (8)
O1^ii—Ti4—O2^i	91.77 (9)	O3^vii—Fe3'—O4	96.35 (10)
O4^iii—Ti4—O4^i	101.06 (9)	O3^vii—Fe3'—O2^vii	84.37 (10)
O4^iii—Ti4—O2^iii	82.96 (8)	O3^viii—Fe3'—O4	96.35 (10)
O4^iii—Ti4—O2^i	174.90 (9)	O3^viii—Fe3'—O2^vii	84.37 (10)
O4^i—Ti4—O2^iii	174.90 (9)	O4—Fe3'—O2^vii	178.97 (12)
O4^i—Ti4—O2^i	82.96 (8)	O5^i—Mg4—O1^ii	172.99 (10)
O2^iii—Ti4—O2^i	92.86 (8)	O5^i—Mg4—O4^iii	98.94 (9)
O5^i—Al4—O1^ii	172.99 (10)	O5^i—Mg4—O4^i	98.94 (9)
O5^i—Al4—O4^iii	98.94 (9)	O5^i—Mg4—O2^iii	83.42 (9)
O5^i—Al4—O4^i	98.94 (9)	O5^i—Mg4—O2^i	83.42 (9)
O5^i—Al4—O2^iii	83.42 (9)	O1^ii—Mg4—O4^iii	85.47 (9)
O5^i—Al4—O2^i	83.42 (9)	O1^ii—Mg4—O4^i	85.47 (9)
O1^ii—Al4—O4^iii	85.47 (9)	O1^ii—Mg4—O2^iii	91.77 (9)
O1^ii—Al4—O4^i	85.47 (9)	O1^ii—Mg4—O2^i	91.77 (9)
O1^ii—Al4—O2^iii	91.77 (9)	O4^iii—Mg4—O4^i	101.06 (9)
O1^ii—Al4—O2^i	91.77 (9)	O4^iii—Mg4—O2^iii	82.96 (8)
O4^iii—Al4—O4^i	101.06 (9)	O4^iii—Mg4—O2^i	174.90 (9)
O4^iii—Al4—O2^iii	82.96 (8)	O4^i—Mg4—O2^iii	174.90 (9)
O4^iii—Al4—O2^i	174.90 (9)	O4^i—Mg4—O2^i	82.96 (8)
O4^i—Al4—O2^iii	174.90 (9)	O2^iii—Mg4—O2^i	92.86 (8)
O4^i—Al4—O2^i	82.96 (8)	O1^ix—Mg1—O1^x	91.01 (7)
O2^iii—Al4—O2^i	92.86 (8)	O1^ix—Mg1—O1^xi	88.99 (7)
O3—Mg2—O3^iv	180.0 (5)	O1^ix—Mg1—O1^iv	180.0 (5)
O3—Mg2—O2	85.06 (9)	O1^ix—Mg1—O4	96.47 (9)
O3—Mg2—O2^v	85.06 (9)	O1^ix—Mg1—O4^xii	83.53 (9)
O3—Mg2—O2^iv	94.94 (9)	O1^x—Mg1—O1^xi	180.0 (5)
O3—Mg2—O2^vi	94.94 (9)	O1^x—Mg1—O1^iv	88.99 (7)
O3^iv—Mg2—O2	94.94 (9)	O1^x—Mg1—O4	96.47 (9)
O3^iv—Mg2—O2^v	94.94 (9)	O1^x—Mg1—O4^xii	83.53 (9)
O3^iv—Mg2—O2^iv	85.06 (9)	O1^xi—Mg1—O1^iv	91.01 (7)
O3^iv—Mg2—O2^vi	85.06 (9)	O1^xi—Mg1—O4	83.53 (9)
O2—Mg2—O2^v	93.13 (7)	O1^xi—Mg1—O4^xii	96.47 (9)
O2—Mg2—O2^iv	86.87 (7)	O1^iv—Mg1—O4	83.53 (9)
O2—Mg2—O2^vi	180.0 (5)	O1^iv—Mg1—O4^xii	96.47 (9)
O2^v—Mg2—O2^iv	180.0 (5)	O4—Mg1—O4^xii	180.0 (5)
O2^v—Mg2—O2^vi	86.87 (7)	O5—B1—O3^viii	118.9 (3)
O2^iv—Mg2—O2^vi	93.13 (7)	O5—B1—O1^viii	118.9 (3)
O5^vii—Mg3—O5^viii	90.27 (8)	O3^viii—B1—O1^viii	122.3 (3)
O5^vii—Mg3—O3^vii	88.03 (7)

Symmetry codes: (i) x+1/2, −y+1/2, −z+1; (ii) −x+3/2, y+1/2, −z+1; (iii) x+1/2, −y+1/2, −z; (iv) −x+1, −y, z; (v) x, y, z+1; (vi) −x+1, −y, z+1; (vii) x−1/2, −y+1/2, −z; (viii) x−1/2, −y+1/2, −z+1; (ix) x−1, y, z−1; (x) x−1, y, z; (xi) −x+1, −y, z−1; (xii) −x, −y, z.