A structural phase transition in NaTaOGeO₄ and its relation to phase transitions in titanite

A structural phase transition from space-group symmetry P2₁/c to C2/c is reported for NaTaOGeO₄ (NTGO). The critical temperature has been located at T_c = 116 K, based on the appearance of sharp diffraction maxima at positions h + k = 2n + 1 of reciprocal space on cooling below this temperature. Strongly anisotropic diffuse scattering in sheets normal to [001] is observable for T > T_c and persists up to ambient temperature. Similarities to phase transitions observed in other compounds of the titanite structure type are discussed. The symmetry properties of these phase transitions are reassessed on the basis of the structural data available. The primary order parameter is identified with the displacement of the transition metal cation M (M = Ta in NTGO) away from the centre of symmetry that it nominally occupies in the paraphase. The order parameter transforms as the Y representation. The anisotropic diffuse scattering is attributed to the one-dimensional correlation of local M displacements parallel to the direction of chains of trans-corner-sharing MO₆ octahedra. The critical temperatures of the isomorphous phase transitions in various titanite-type compounds depend linearly on the squared transition-metal displacement measured in the ordered P2₁/c phase.

Computing details

For both compounds, program(s) used to refine structure: (Jana2000; Petricek and Dusek, 2000); software used to prepare material for publication: (Jana2000; Petricek and Dusek, 2000).

(1)

Crystal data

GeNaO5Ta	Z = 4
Mr = 356.5	F(000) = 624
Monoclinic, P21/c	Dx = 5.760 Mg m^−3
Hall symbol: -P 2ybc	Synchrotron radiation, λ = 0.39184 Å
a = 6.838 (1) Å	µ = 6.97 mm^−1
b = 8.930 (1) Å	T = 98 K
c = 7.414 (1) Å	Tabular, colourless
β = 114.800 (6)°	0.1 × 0.09 × 0.05 mm
V = 411.0 (1) Å^3

Data collection

Beamline F1 at Hasylab diffractometer	Rint = 0.044
φ–scan	θmax = 45.9°, θmin = 1.8°
95016 measured reflections	h = −25→25
20505 independent reflections	k = −32→27
17954 reflections with I > 3σ(I)	l = −27→23

Refinement

Refinement on F^2	Weighting scheme based on measured s.u.'s w = 1/(σ^2(I) + 0.0016I^2)
R[F^2 > 2σ(F^2)] = 0.023	(Δ/σ)max = 0.012
wR(F^2) = 0.073	Δρmax = 2.95 e Å^−3
S = 1.06	Δρmin = −4.68 e Å^−3
20505 reflections	Extinction correction: B-C type 1 Gaussian isotropic (Becker & Coppens, 1974)
74 parameters	Extinction coefficient: 0.00345 (16)

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

	x	y	z	Uiso*/Ueq
Na	0.24827 (4)	0.92207 (6)	0.24235 (5)	0.01108 (10)
Ta	0.748245 (2)	0.752060 (2)	0.504007 (3)	0.003180 (5)
Ge	0.249340 (6)	0.567282 (8)	0.249414 (6)	0.003072 (8)
O1	0.25058 (5)	0.18307 (5)	0.24884 (4)	0.00508 (5)
O2	0.44387 (6)	0.68888 (4)	0.41119 (6)	0.00602 (5)
O3	0.94724 (6)	0.18710 (4)	0.41330 (6)	0.00594 (5)
O4	0.35039 (6)	0.45542 (4)	0.11373 (6)	0.00544 (5)
O5	0.84765 (6)	0.95470 (4)	0.11214 (6)	0.00540 (5)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Na	0.00875 (12)	0.00526 (9)	0.01730 (19)	−0.00006 (5)	0.00358 (12)	−0.00022 (7)
Ta	0.002967 (6)	0.003137 (6)	0.003265 (7)	−0.000264 (2)	0.001138 (5)	0.000239 (2)
Ge	0.00288 (1)	0.00298 (1)	0.00323 (1)	0.000007 (5)	0.001150 (8)	−0.000005 (6)
O1	0.00648 (8)	0.00553 (8)	0.00360 (6)	0.00003 (4)	0.00248 (6)	0.00003 (5)
O2	0.00439 (7)	0.00633 (8)	0.00681 (8)	−0.00152 (6)	0.00183 (6)	−0.00164 (7)
O3	0.00457 (7)	0.00619 (8)	0.00663 (8)	−0.00150 (6)	0.00192 (6)	−0.00151 (7)
O4	0.00622 (7)	0.00451 (7)	0.00654 (8)	−0.00052 (5)	0.00360 (6)	−0.00135 (6)
O5	0.00621 (7)	0.00440 (6)	0.00646 (8)	−0.00048 (5)	0.00351 (6)	−0.00122 (6)

Geometric parameters (Å, º)

Na—O1^i	2.3312 (7)	O1—O2^viii	2.8027 (6)
Na—O2	2.5075 (6)	O1—O3^iv	2.8129 (6)
Na—O3^ii	2.4956 (6)	O1—O3^ix	2.7398 (4)
Na—O4^ii	2.5104 (5)	O1—O4	2.8210 (6)
Na—O4^iii	2.7664 (6)	O1—O4^x	2.7866 (5)
Na—O5^iv	2.5106 (5)	O1—O5^xi	2.7604 (5)
Na—O5^v	2.6651 (6)	O1—O5^viii	2.8245 (6)
Ta—O1^vi	1.9186 (3)	O2—O3^ii	2.7465 (4)
Ta—O1^ii	1.9765 (3)	O2—O4	2.9045 (6)
Ta—O2	1.9805 (4)	O2—O4^ii	2.8101 (6)
Ta—O3^vii	1.9839 (4)	O2—O5^viii	2.8438 (6)
Ta—O4^ii	2.0052 (4)	O2—O5^iii	2.8348 (5)
Ta—O5^iii	2.0141 (4)	O3—O4^viii	2.8496 (6)
Ge—O2	1.7461 (4)	O3—O4^xii	2.8268 (5)
Ge—O3^ii	1.7455 (4)	O3—O5^xiii	2.9121 (6)
Ge—O4	1.7523 (5)	O3—O5^xiv	2.8170 (6)
Ge—O5^viii	1.7536 (5)	O4—O5^viii	2.8763 (7)
O1—O2^vi	2.7582 (4)
O1^i—Na—O2	145.328 (16)	Na^viii—O3—O5^xiii	109.695 (17)
O1^i—Na—O3^ii	147.984 (16)	Na^viii—O3—O5^xiv	56.010 (14)
O1^i—Na—O4^ii	82.870 (17)	Ta^vii—O3—Ge^viii	141.78 (3)
O1^i—Na—O4^iii	65.630 (16)	Ta^vii—O3—O1^xv	42.965 (11)
O1^i—Na—O5^iv	83.612 (18)	Ta^vii—O3—O1^xii	46.115 (11)
O1^i—Na—O5^v	66.688 (17)	Ta^vii—O3—O2^viii	141.19 (2)
O2—Na—O3^ii	66.590 (18)	Ta^vii—O3—O4^viii	147.848 (19)
O2—Na—O4^ii	68.114 (15)	Ta^vii—O3—O4^xii	45.181 (11)
O2—Na—O4^iii	88.420 (17)	Ta^vii—O3—O5^xiii	108.04 (2)
O2—Na—O5^iv	123.77 (2)	Ta^vii—O3—O5^xiv	45.641 (11)
O2—Na—O5^v	133.97 (2)	Ge^viii—O3—O1^xv	100.76 (2)
O3^ii—Na—O4^ii	125.06 (2)	Ge^viii—O3—O1^xii	163.18 (3)
O3^ii—Na—O4^iii	130.95 (2)	Ge^viii—O3—O2^viii	38.138 (11)
O3^ii—Na—O5^iv	68.486 (15)	Ge^viii—O3—O4^viii	35.521 (13)
O3^ii—Na—O5^v	91.641 (17)	Ge^viii—O3—O4^xii	112.696 (19)
O4^ii—Na—O4^iii	75.846 (16)	Ge^viii—O3—O5^xiii	33.758 (13)
O4^ii—Na—O5^iv	166.33 (3)	Ge^viii—O3—O5^xiv	137.26 (2)
O4^ii—Na—O5^v	98.23 (2)	O1^xv—O3—O1^xii	89.077 (15)
O4^iii—Na—O4^ii	75.846 (16)	O1^xv—O3—O2^viii	104.155 (19)
O4^iii—Na—O5^iv	96.660 (19)	O1^xv—O3—O4^viii	126.51 (2)
O4^iii—Na—O5^v	132.32 (2)	O1^xv—O3—O4^xii	59.220 (15)
O5^iv—Na—O5^v	78.193 (16)	O1^xv—O3—O5^xiii	68.675 (16)
O5^v—Na—O5^iv	78.193 (16)	O1^xv—O3—O5^xiv	60.225 (16)
O1^vi—Ta—O1^ii	179.273 (16)	O1^xii—O3—O1^xv	89.077 (15)
O1^vi—Ta—O2	90.036 (16)	O1^xii—O3—O2^viii	151.77 (2)
O1^vi—Ta—O3^vii	92.224 (16)	O1^xii—O3—O4^viii	128.13 (2)
O1^vi—Ta—O4^ii	90.470 (18)	O1^xii—O3—O4^xii	60.873 (13)
O1^vi—Ta—O5^iii	91.782 (18)	O1^xii—O3—O5^xiii	147.377 (19)
O1^ii—Ta—O1^vi	179.272 (16)	O1^xii—O3—O5^xiv	59.552 (14)
O1^ii—Ta—O2	90.196 (16)	O2^viii—O3—O4^viii	62.501 (14)
O1^ii—Ta—O3^vii	87.545 (16)	O2^viii—O3—O4^xii	147.11 (2)
O1^ii—Ta—O4^ii	90.220 (18)	O2^viii—O3—O5^xiii	60.256 (14)
O1^ii—Ta—O5^iii	87.527 (18)	O2^viii—O3—O5^xiv	105.406 (17)
O2—Ta—O3^vii	177.739 (19)	O4^viii—O3—O4^xii	102.731 (16)
O2—Ta—O4^ii	89.665 (16)	O4^viii—O3—O5^xiii	59.885 (15)
O2—Ta—O5^iii	90.411 (16)	O4^viii—O3—O5^xiv	166.447 (17)
O3^vii—Ta—O4^ii	90.246 (16)	O4^xii—O3—O4^viii	102.731 (16)
O3^vii—Ta—O5^iii	89.589 (16)	O4^xii—O3—O5^xiii	86.850 (16)
O4^ii—Ta—O5^iii	177.747 (19)	O4^xii—O3—O5^xiv	90.801 (16)
O2—Ge—O3^ii	103.739 (17)	O5^xiii—O3—O5^xiv	121.07 (2)
O2—Ge—O4	112.25 (2)	O5^xiv—O3—O5^xiii	121.07 (2)
O2—Ge—O5^viii	108.703 (19)	Na^viii—O4—Na^xvi	104.15 (2)
O3^ii—Ge—O4	109.12 (2)	Na^viii—O4—Ta^viii	100.70 (2)
O3^ii—Ge—O5^viii	112.66 (2)	Na^viii—O4—Ge	116.593 (19)
O4—Ge—O5^viii	110.26 (2)	Na^viii—O4—O1	97.525 (17)
Na^xiii—O1—Ta^vi	108.59 (2)	Na^viii—O4—O1^xvii	97.99 (2)
Na^xiii—O1—Ta^viii	107.142 (19)	Na^viii—O4—O2	84.793 (16)
Na^xiii—O1—O2^vi	115.425 (16)	Na^viii—O4—O2^viii	55.895 (16)
Na^xiii—O1—O2^viii	90.548 (18)	Na^viii—O4—O3^ii	137.49 (2)
Na^xiii—O1—O3^iv	91.254 (18)	Na^viii—O4—O3^ix	145.21 (2)
Na^xiii—O1—O3^ix	114.101 (16)	Na^viii—O4—O5^viii	117.010 (18)
Na^xiii—O1—O4	148.78 (2)	Na^xvi—O4—Na^viii	104.15 (2)
Na^xiii—O1—O4^x	64.728 (16)	Na^xvi—O4—Ta^viii	91.518 (18)
Na^xiii—O1—O5^xi	62.456 (16)	Na^xvi—O4—Ge	110.504 (19)
Na^xiii—O1—O5^viii	149.95 (2)	Na^xvi—O4—O1	134.164 (17)
Ta^vi—O1—Ta^viii	144.26 (3)	Na^xvi—O4—O1^xvii	49.643 (16)
Ta^vi—O1—O2^vi	45.891 (12)	Na^xvi—O4—O2	110.804 (18)
Ta^vi—O1—O2^viii	133.909 (16)	Na^xvi—O4—O2^viii	101.18 (2)
Ta^vi—O1—O3^iv	44.811 (11)	Na^xvi—O4—O3^ii	76.598 (16)
Ta^vi—O1—O3^ix	115.96 (2)	Na^xvi—O4—O3^ix	82.615 (15)
Ta^vi—O1—O4	100.028 (19)	Na^xvi—O4—O5^viii	134.996 (18)
Ta^vi—O1—O4^x	46.020 (11)	Ta^viii—O4—Ge	128.65 (3)
Ta^vi—O1—O5^xi	164.24 (2)	Ta^viii—O4—O1	44.477 (11)
Ta^vi—O1—O5^viii	45.459 (13)	Ta^viii—O4—O1^xvii	43.510 (12)
Ta^viii—O1—Ta^vi	144.26 (3)	Ta^viii—O4—O2	155.13 (2)
Ta^viii—O1—O2^vi	115.762 (19)	Ta^viii—O4—O2^viii	44.809 (11)
Ta^viii—O1—O2^viii	44.960 (10)	Ta^viii—O4—O3^ii	121.802 (18)
Ta^viii—O1—O3^iv	134.770 (15)	Ta^viii—O4—O3^ix	44.572 (11)
Ta^viii—O1—O3^ix	46.340 (12)	Ta^viii—O4—O5^viii	97.670 (19)
Ta^viii—O1—O4	45.302 (12)	Ge—O4—O1	94.31 (2)
Ta^viii—O1—O4^x	164.23 (2)	Ge—O4—O1^xvii	144.624 (19)
Ta^viii—O1—O5^xi	46.801 (11)	Ge—O4—O2	33.807 (11)
Ta^viii—O1—O5^viii	100.056 (19)	Ge—O4—O2^viii	148.15 (2)
O2^vi—O1—O2^viii	88.036 (15)	Ge—O4—O3^ii	35.363 (11)
O2^vi—O1—O3^iv	90.701 (16)	Ge—O4—O3^ix	91.49 (2)
O2^vi—O1—O3^ix	130.47 (2)	Ge—O4—O5^viii	34.887 (12)
O2^vi—O1—O4	76.333 (15)	O1—O4—O1^xvii	87.987 (15)
O2^vi—O1—O4^x	60.899 (13)	O1—O4—O2	110.99 (2)
O2^vi—O1—O5^xi	148.624 (19)	O1—O4—O2^viii	59.699 (14)
O2^vi—O1—O5^viii	61.018 (13)	O1—O4—O3^ii	112.26 (2)
O2^viii—O1—O2^vi	88.036 (15)	O1—O4—O3^ix	58.039 (12)
O2^viii—O1—O3^iv	178.12 (2)	O1—O4—O5^viii	59.431 (14)
O2^viii—O1—O3^ix	91.300 (16)	O1^xvii—O4—O1	87.987 (15)
O2^viii—O1—O4	59.958 (15)	O1^xvii—O4—O2	160.39 (2)
O2^viii—O1—O4^x	119.781 (16)	O1^xvii—O4—O2^viii	59.052 (14)
O2^viii—O1—O5^xi	61.265 (14)	O1^xvii—O4—O3^ii	112.153 (15)
O2^viii—O1—O5^viii	118.176 (19)	O1^xvii—O4—O3^ix	60.141 (14)
O3^iv—O1—O3^ix	88.458 (16)	O1^xvii—O4—O5^viii	133.534 (18)
O3^iv—O1—O4	118.369 (19)	O2—O4—O2^viii	134.516 (16)
O3^iv—O1—O4^x	60.639 (14)	O2—O4—O3^ii	57.009 (13)
O3^iv—O1—O5^xi	120.145 (16)	O2—O4—O3^ix	125.29 (2)
O3^iv—O1—O5^viii	59.959 (15)	O2—O4—O5^viii	58.935 (15)
O3^ix—O1—O3^iv	88.458 (16)	O2^viii—O4—O2	134.516 (16)
O3^ix—O1—O4	61.088 (14)	O2^viii—O4—O3^ii	166.606 (17)
O3^ix—O1—O4^x	148.56 (2)	O2^viii—O4—O3^ix	89.360 (15)
O3^ix—O1—O5^xi	61.615 (13)	O2^viii—O4—O5^viii	116.208 (18)
O3^ix—O1—O5^viii	76.265 (15)	O3^ii—O4—O3^ix	77.269 (15)
O4—O1—O4^x	136.900 (16)	O3^ii—O4—O5^viii	61.136 (15)
O4—O1—O5^xi	92.104 (16)	O3^ix—O4—O3^ii	77.269 (15)
O4—O1—O5^viii	61.260 (17)	O3^ix—O4—O5^viii	74.101 (17)
O4^x—O1—O4	136.900 (16)	Na^xv—O5—Na^v	101.81 (2)
O4^x—O1—O5^xi	127.18 (2)	Na^xv—O5—Ta^xvi	100.27 (2)
O4^x—O1—O5^viii	91.478 (16)	Na^xv—O5—Ge^ii	117.456 (19)
O5^xi—O1—O5^viii	137.574 (16)	Na^xv—O5—O1^xi	95.95 (2)
O5^viii—O1—O5^xi	137.574 (16)	Na^xv—O5—O1^ii	98.521 (18)
Na—O2—Ta	101.516 (18)	Na^xv—O5—O2^ii	137.11 (2)
Na—O2—Ge	94.611 (16)	Na^xv—O5—O2^xvi	144.56 (2)
Na—O2—O1^vi	98.794 (18)	Na^xv—O5—O3^i	85.490 (16)
Na—O2—O1^ii	98.06 (2)	Na^xv—O5—O3^xviii	55.504 (16)
Na—O2—O3^ii	56.496 (14)	Na^xv—O5—O4^ii	119.326 (18)
Na—O2—O4	108.397 (17)	Na^v—O5—Na^xv	101.81 (2)
Na—O2—O4^ii	55.991 (14)	Na^v—O5—Ta^xvi	94.795 (18)
Na—O2—O5^viii	111.519 (16)	Na^v—O5—Ge^ii	110.087 (19)
Na—O2—O5^iii	146.72 (2)	Na^v—O5—O1^xi	50.857 (16)
Ta—O2—Ge	143.38 (3)	Na^v—O5—O1^ii	135.776 (18)
Ta—O2—O1^vi	44.073 (10)	Na^v—O5—O2^ii	76.118 (16)
Ta—O2—O1^ii	44.844 (12)	Na^v—O5—O2^xvi	83.448 (15)
Ta—O2—O3^ii	143.26 (2)	Na^v—O5—O3^i	110.093 (19)
Ta—O2—O4	109.44 (2)	Na^v—O5—O3^xviii	101.99 (2)
Ta—O2—O4^ii	45.525 (11)	Na^v—O5—O4^ii	134.574 (18)
Ta—O2—O5^viii	146.961 (19)	Ta^xvi—O5—Ge^ii	127.68 (3)
Ta—O2—O5^iii	45.274 (11)	Ta^xvi—O5—O1^xi	45.671 (12)
Ge—O2—O1^vi	162.24 (2)	Ta^xvi—O5—O1^ii	42.760 (10)
Ge—O2—O1^ii	100.72 (2)	Ta^xvi—O5—O2^ii	122.614 (18)
Ge—O2—O3^ii	38.123 (11)	Ta^xvi—O5—O2^xvi	44.315 (11)
Ge—O2—O4	33.944 (13)	Ta^xvi—O5—O3^i	152.85 (2)
Ge—O2—O4^ii	137.71 (2)	Ta^xvi—O5—O3^xviii	44.769 (11)
Ge—O2—O5^viii	35.737 (13)	Ta^xvi—O5—O4^ii	95.912 (18)
Ge—O2—O5^iii	111.910 (19)	Ge^ii—O5—O1^xi	145.594 (19)
O1^vi—O2—O1^ii	88.916 (15)	Ge^ii—O5—O1^ii	94.16 (2)
O1^vi—O2—O3^ii	152.74 (2)	Ge^ii—O5—O2^ii	35.560 (11)
O1^vi—O2—O4	146.358 (19)	Ge^ii—O5—O2^xvi	92.45 (2)
O1^vi—O2—O4^ii	60.048 (14)	Ge^ii—O5—O3^i	33.581 (11)
O1^vi—O2—O5^viii	127.11 (2)	Ge^ii—O5—O3^xviii	147.86 (2)
O1^vi—O2—O5^iii	60.645 (13)	Ge^ii—O5—O4^ii	34.856 (12)
O1^ii—O2—O1^vi	88.916 (15)	O1^xi—O5—O1^ii	88.431 (15)
O1^ii—O2—O3^ii	104.713 (19)	O1^xi—O5—O2^ii	112.922 (15)
O1^ii—O2—O4	68.345 (16)	O1^xi—O5—O2^xvi	60.104 (14)
O1^ii—O2—O4^ii	60.343 (16)	O1^xi—O5—O3^i	160.83 (2)
O1^ii—O2—O5^viii	126.06 (2)	O1^xi—O5—O3^xviii	58.833 (14)
O1^ii—O2—O5^iii	58.631 (14)	O1^xi—O5—O4^ii	133.697 (18)
O3^ii—O2—O4	60.489 (14)	O1^ii—O5—O1^xi	88.431 (15)
O3^ii—O2—O4^ii	106.124 (17)	O1^ii—O5—O2^ii	112.69 (2)
O3^ii—O2—O5^viii	62.759 (14)	O1^ii—O5—O2^xvi	58.337 (12)
O3^ii—O2—O5^iii	146.48 (2)	O1^ii—O5—O3^i	110.313 (19)
O4—O2—O4^ii	120.99 (2)	O1^ii—O5—O3^xviii	59.815 (14)
O4—O2—O5^viii	60.039 (15)	O1^ii—O5—O4^ii	59.308 (14)
O4—O2—O5^iii	85.988 (16)	O2^ii—O5—O2^xvi	78.305 (15)
O4^ii—O2—O4	120.99 (2)	O2^ii—O5—O3^i	56.984 (13)
O4^ii—O2—O5^viii	167.507 (17)	O2^ii—O5—O3^xviii	167.348 (17)
O4^ii—O2—O5^iii	90.777 (15)	O2^ii—O5—O4^ii	61.027 (15)
O5^viii—O2—O5^iii	101.695 (16)	O2^xvi—O5—O2^ii	78.305 (15)
O5^iii—O2—O5^viii	101.695 (16)	O2^xvi—O5—O3^i	126.03 (2)
Na^viii—O3—Ta^vii	101.647 (19)	O2^xvi—O5—O3^xviii	89.062 (15)
Na^viii—O3—Ge^viii	95.044 (16)	O2^xvi—O5—O4^ii	74.272 (17)
Na^viii—O3—O1^xv	99.19 (2)	O3^i—O5—O3^xviii	133.870 (15)
Na^viii—O3—O1^xii	96.822 (18)	O3^i—O5—O4^ii	58.979 (15)
Na^viii—O3—O2^viii	56.914 (14)	O3^xviii—O5—O3^i	133.870 (15)
Na^viii—O3—O4^viii	110.465 (17)	O3^xviii—O5—O4^ii	116.386 (18)
Na^viii—O3—O4^xii	146.80 (2)

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

(2)

Crystal data

GeNaO5Ta	Z = 4
Mr = 356.5	F(000) = 624
Monoclinic, C2/c	Dx = 5.745 Mg m^−3
Hall symbol: -C 2yc	Synchrotron radiation, λ = 0.39184 Å
a = 6.854 (1) Å	µ = 6.95 mm^−1
b = 8.933 (1) Å	T = 295 K
c = 7.418 (1) Å	Tabular, colourless
β = 114.858 (2)°	0.1 × 0.09 × 0.05 mm
V = 412.10 (9) Å^3

Data collection

Beamline F1 at Hasylab diffractometer	Rint = 0.048
φ–scan	θmax = 53.3°, θmin = 2.3°
95651 measured reflections	h = −28→28
14190 independent reflections	k = −35→29
11959 reflections with I > 3σ(I)	l = −29→26

Refinement

Refinement on F^2	Weighting scheme based on measured s.u.'s w = 1/(σ^2(I) + 0.0016I^2)
R[F^2 > 2σ(F^2)] = 0.020	(Δ/σ)max = 0.001
wR(F^2) = 0.066	Δρmax = 2.59 e Å^−3
S = 1.02	Δρmin = −2.42 e Å^−3
14190 reflections	Extinction correction: B-C type 1 Gaussian isotropic (Becker & Coppens, 1974)
54 parameters	Extinction coefficient: 0.00728 (19)

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

	x	y	z	Uiso*/Ueq
Na	0	0.32594 (11)	0.75	0.0200 (2)
Ta	0	0	0	0.006843 (5)
Ge	0	0.317582 (8)	0.25	0.006317 (8)
O1	0	0.06634 (6)	0.75	0.00941 (7)
O2	0.19575 (6)	0.43833 (5)	0.41118 (6)	0.01141 (6)
O3	0.09772 (6)	0.20534 (4)	0.11237 (6)	0.01002 (6)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Na	0.0153 (3)	0.0127 (3)	0.0282 (4)	0	0.0053 (3)	0
Ta	0.006618 (6)	0.006617 (7)	0.006871 (7)	0.000740 (2)	0.002422 (4)	−0.000577 (3)
Ge	0.006038 (10)	0.005849 (11)	0.006682 (12)	0	0.002299 (8)	0
O1	0.01312 (12)	0.00965 (9)	0.00612 (7)	0	0.00471 (7)	0
O2	0.00816 (6)	0.01219 (9)	0.01282 (9)	−0.00334 (6)	0.00336 (6)	−0.00369 (8)
O3	0.01225 (8)	0.00757 (6)	0.01229 (8)	−0.00119 (6)	0.00715 (7)	−0.00271 (6)

Geometric parameters (Å, º)

Na—O1	2.3189 (11)	Ge—O3^v	1.7525 (5)
Na—O2^i	2.5130 (9)	O1—O2^xii	2.7508 (4)
Na—O2^ii	2.5130 (9)	O1—O2^iv	2.8064 (5)
Na—O3^iii	2.7058 (6)	O1—O2^xiii	2.7508 (4)
Na—O3^iv	2.5211 (4)	O1—O2^vi	2.8064 (5)
Na—O3^v	2.7057 (6)	O1—O3^iii	2.7753 (5)
Na—O3^vi	2.5211 (4)	O1—O3^viii	2.8206 (6)
Ta—O1^vii	1.9469 (2)	O1—O3^v	2.7753 (5)
Ta—O1^viii	1.9469 (2)	O1—O3^xiv	2.8206 (6)
Ta—O2^ix	1.9827 (4)	O2—O2^v	2.7429 (4)
Ta—O2^x	1.9827 (4)	O2—O3	2.9059 (6)
Ta—O3	2.0096 (3)	O2—O3^v	2.8483 (6)
Ta—O3^xi	2.0096 (3)	O2—O3^xv	2.8162 (6)
Ge—O2	1.7448 (4)	O2—O3^xvi	2.8298 (5)
Ge—O2^v	1.7448 (4)	O3—O3^v	2.8746 (7)
Ge—O3	1.7525 (5)
O1—Na—O2^i	146.925 (14)	O3^xiv—O1—O3^iii	137.578 (15)
O1—Na—O2^ii	146.925 (14)	O3^xiv—O1—O3^viii	61.271 (17)
O1—Na—O3^iii	66.54 (2)	O3^xiv—O1—O3^v	91.816 (13)
O1—Na—O3^iv	83.64 (2)	Na^i—O2—Ta^xv	101.65 (2)
O1—Na—O3^v	66.54 (2)	Na^i—O2—Ge	95.110 (18)
O1—Na—O3^vi	83.64 (2)	Na^i—O2—O1^xvii	97.72 (2)
O2^i—Na—O2^ii	66.15 (3)	Na^i—O2—O1^iv	98.829 (18)
O2^i—Na—O3^iii	131.76 (2)	Na^i—O2—O2^v	56.925 (16)
O2^i—Na—O3^iv	124.04 (3)	Na^i—O2—O3	109.346 (18)
O2^i—Na—O3^v	90.075 (15)	Na^i—O2—O3^v	111.170 (17)
O2^i—Na—O3^vi	68.032 (16)	Na^i—O2—O3^xv	56.121 (15)
O2^ii—Na—O2^i	66.15 (3)	Na^i—O2—O3^xvi	146.89 (2)
O2^ii—Na—O3^iii	90.075 (15)	Ta^xv—O2—Ge	142.93 (3)
O2^ii—Na—O3^iv	68.032 (16)	Ta^xv—O2—O1^xvii	45.041 (7)
O2^ii—Na—O3^v	131.76 (2)	Ta^xv—O2—O1^iv	43.915 (11)
O2^ii—Na—O3^vi	124.04 (3)	Ta^xv—O2—O2^v	142.80 (3)
O3^iii—Na—O3^iv	77.393 (16)	Ta^xv—O2—O3	109.03 (2)
O3^iii—Na—O3^v	133.07 (4)	Ta^xv—O2—O3^v	147.18 (2)
O3^iii—Na—O3^vi	97.472 (18)	Ta^xv—O2—O3^xv	45.526 (12)
O3^iv—Na—O3^iii	77.393 (16)	Ta^xv—O2—O3^xvi	45.246 (11)
O3^iv—Na—O3^v	97.472 (18)	Ge—O2—O1^xvii	162.23 (3)
O3^iv—Na—O3^vi	167.28 (4)	Ge—O2—O1^iv	101.18 (2)
O3^v—Na—O3^iii	133.07 (4)	Ge—O2—O2^v	38.185 (11)
O3^v—Na—O3^iv	97.472 (18)	Ge—O2—O3	33.892 (13)
O3^v—Na—O3^vi	77.393 (16)	Ge—O2—O3^v	35.558 (13)
O3^vi—Na—O3^iii	97.472 (18)	Ge—O2—O3^xv	137.97 (2)
O3^vi—Na—O3^iv	167.28 (4)	Ge—O2—O3^xvi	112.26 (2)
O3^vi—Na—O3^v	77.393 (16)	O1^xvii—O2—O1^iv	88.956 (11)
O1^vii—Ta—O1^viii	180	O1^xvii—O2—O2^v	152.20 (2)
O1^vii—Ta—O2^ix	88.853 (15)	O1^xvii—O2—O3	146.938 (18)
O1^vii—Ta—O2^x	91.147 (15)	O1^xvii—O2—O3^v	127.18 (2)
O1^vii—Ta—O3	89.072 (19)	O1^xvii—O2—O3^xv	59.790 (13)
O1^vii—Ta—O3^xi	90.928 (19)	O1^xvii—O2—O3^xvi	60.699 (13)
O1^viii—Ta—O1^vii	180	O1^iv—O2—O1^xvii	88.956 (11)
O1^viii—Ta—O2^ix	91.147 (15)	O1^iv—O2—O2^v	104.976 (19)
O1^viii—Ta—O2^x	88.853 (15)	O1^iv—O2—O3	68.860 (17)
O1^viii—Ta—O3	90.928 (19)	O1^iv—O2—O3^v	126.52 (2)
O1^viii—Ta—O3^xi	89.072 (19)	O1^iv—O2—O3^xv	60.217 (17)
O2^ix—Ta—O2^x	180	O1^iv—O2—O3^xvi	58.995 (14)
O2^ix—Ta—O3	89.724 (17)	O2^v—O2—O3	60.479 (14)
O2^ix—Ta—O3^xi	90.276 (17)	O2^v—O2—O3^v	62.595 (14)
O2^x—Ta—O2^ix	180	O2^v—O2—O3^xv	106.199 (18)
O2^x—Ta—O3	90.276 (17)	O2^v—O2—O3^xvi	146.99 (2)
O2^x—Ta—O3^xi	89.724 (17)	O3—O2—O3^v	59.932 (16)
O3—Ta—O3^xi	180	O3—O2—O3^xv	121.36 (2)
O3^xi—Ta—O3	180	O3—O2—O3^xvi	86.510 (16)
O2—Ge—O2^v	103.630 (18)	O3^v—O2—O3	59.932 (16)
O2—Ge—O3	112.38 (2)	O3^v—O2—O3^xv	167.287 (17)
O2—Ge—O3^v	109.06 (2)	O3^v—O2—O3^xvi	101.935 (17)
O2^v—Ge—O2	103.630 (18)	O3^xv—O2—O3	121.36 (2)
O2^v—Ge—O3	109.06 (2)	O3^xv—O2—O3^v	167.287 (17)
O2^v—Ge—O3^v	112.38 (2)	O3^xv—O2—O3^xvi	90.772 (15)
O3—Ge—O3^v	110.199 (19)	O3^xvi—O2—O3	86.510 (16)
O3^v—Ge—O3	110.199 (19)	O3^xvi—O2—O3^v	101.935 (17)
Na—O1—Ta^iii	107.722 (14)	O3^xvi—O2—O3^xv	90.772 (15)
Na—O1—Ta^v	107.722 (14)	Na^vii—O3—Na^iv	102.607 (19)
Na—O1—O2^xii	114.564 (13)	Na^vii—O3—Ta	92.94 (2)
Na—O1—O2^iv	90.852 (13)	Na^vii—O3—Ge	110.81 (2)
Na—O1—O2^xiii	114.564 (13)	Na^vii—O3—O1^vii	50.04 (2)
Na—O1—O2^vi	90.852 (13)	Na^vii—O3—O1^viii	134.82 (2)
Na—O1—O3^iii	63.424 (12)	Na^vii—O3—O2	110.73 (2)
Na—O1—O3^viii	149.364 (11)	Na^vii—O3—O2^v	76.976 (18)
Na—O1—O3^v	63.424 (12)	Na^vii—O3—O2^ix	101.34 (2)
Na—O1—O3^xiv	149.364 (11)	Na^vii—O3—O2^x	82.941 (17)
Ta^iii—O1—Ta^v	144.56 (3)	Na^vii—O3—O3^v	135.319 (18)
Ta^iii—O1—O2^xii	116.119 (18)	Na^iv—O3—Na^vii	102.607 (19)
Ta^iii—O1—O2^iv	134.332 (12)	Na^iv—O3—Ta	100.60 (3)
Ta^iii—O1—O2^xiii	46.106 (11)	Na^iv—O3—Ge	116.63 (2)
Ta^iii—O1—O2^vi	44.938 (7)	Na^iv—O3—O1^vii	96.905 (19)
Ta^iii—O1—O3^iii	46.387 (9)	Na^iv—O3—O1^viii	98.27 (2)
Ta^iii—O1—O3^viii	45.429 (12)	Na^iv—O3—O2	84.70 (2)
Ta^iii—O1—O3^v	164.187 (16)	Na^iv—O3—O2^v	136.86 (3)
Ta^iii—O1—O3^xiv	100.25 (2)	Na^iv—O3—O2^ix	55.85 (2)
Ta^v—O1—Ta^iii	144.56 (3)	Na^iv—O3—O2^x	145.07 (3)
Ta^v—O1—O2^xii	46.106 (11)	Na^iv—O3—O3^v	117.982 (15)
Ta^v—O1—O2^iv	44.938 (7)	Ta—O3—Ge	128.43 (3)
Ta^v—O1—O2^xiii	116.119 (18)	Ta—O3—O1^vii	44.541 (12)
Ta^v—O1—O2^vi	134.332 (12)	Ta—O3—O1^viii	43.642 (9)
Ta^v—O1—O3^iii	164.187 (16)	Ta—O3—O2	154.16 (2)
Ta^v—O1—O3^viii	100.25 (2)	Ta—O3—O2^v	122.541 (18)
Ta^v—O1—O3^v	46.387 (9)	Ta—O3—O2^ix	44.750 (10)
Ta^v—O1—O3^xiv	45.429 (12)	Ta—O3—O2^x	44.478 (11)
O2^xii—O1—O2^iv	91.044 (14)	Ta—O3—O3^v	96.953 (18)
O2^xii—O1—O2^xiii	130.87 (2)	Ge—O3—O1^vii	145.540 (18)
O2^xii—O1—O2^vi	88.247 (14)	Ge—O3—O1^viii	94.26 (2)
O2^xii—O1—O3^iii	148.491 (14)	Ge—O3—O2	33.724 (10)
O2^xii—O1—O3^viii	76.638 (16)	Ge—O3—O2^v	35.379 (12)
O2^xii—O1—O3^v	61.276 (12)	Ge—O3—O2^ix	147.76 (2)
O2^xii—O1—O3^xiv	61.036 (14)	Ge—O3—O2^x	92.35 (2)
O2^iv—O1—O2^xii	91.044 (14)	Ge—O3—O3^v	34.901 (12)
O2^iv—O1—O2^xiii	88.247 (14)	O1^vii—O3—O1^viii	88.184 (14)
O2^iv—O1—O2^vi	178.30 (2)	O1^vii—O3—O2	160.65 (2)
O2^iv—O1—O3^iii	119.953 (14)	O1^vii—O3—O2^v	113.008 (15)
O2^iv—O1—O3^viii	118.26 (2)	O1^vii—O3—O2^ix	58.934 (14)
O2^iv—O1—O3^v	60.923 (12)	O1^vii—O3—O2^x	60.082 (13)
O2^iv—O1—O3^xiv	60.064 (14)	O1^vii—O3—O3^v	133.886 (16)
O2^xiii—O1—O2^xii	130.87 (2)	O1^viii—O3—O1^vii	88.184 (14)
O2^xiii—O1—O2^iv	88.247 (14)	O1^viii—O3—O2	110.739 (19)
O2^xiii—O1—O2^vi	91.044 (14)	O1^viii—O3—O2^v	112.45 (2)
O2^xiii—O1—O3^iii	61.276 (12)	O1^viii—O3—O2^ix	59.719 (13)
O2^xiii—O1—O3^viii	61.036 (14)	O1^viii—O3—O2^x	58.265 (12)
O2^xiii—O1—O3^v	148.490 (14)	O1^viii—O3—O3^v	59.364 (13)
O2^xiii—O1—O3^xiv	76.638 (16)	O2—O3—O2^v	56.926 (13)
O2^vi—O1—O2^xii	88.247 (14)	O2—O3—O2^ix	133.854 (16)
O2^vi—O1—O2^iv	178.30 (2)	O2—O3—O2^x	126.07 (2)
O2^vi—O1—O2^xiii	91.044 (14)	O2—O3—O3^v	59.039 (15)
O2^vi—O1—O3^iii	60.923 (12)	O2^v—O3—O2	56.926 (13)
O2^vi—O1—O3^viii	60.064 (14)	O2^v—O3—O2^ix	167.287 (17)
O2^vi—O1—O3^v	119.953 (14)	O2^v—O3—O2^x	78.065 (15)
O2^vi—O1—O3^xiv	118.26 (2)	O2^v—O3—O3^v	61.029 (16)
O3^iii—O1—O3^viii	91.816 (13)	O2^ix—O3—O2	133.854 (16)
O3^iii—O1—O3^v	126.85 (2)	O2^ix—O3—O2^v	167.287 (17)
O3^iii—O1—O3^xiv	137.578 (15)	O2^ix—O3—O2^x	89.228 (15)
O3^viii—O1—O3^iii	91.816 (13)	O2^ix—O3—O3^v	116.137 (18)
O3^viii—O1—O3^v	137.578 (15)	O2^x—O3—O2	126.07 (2)
O3^viii—O1—O3^xiv	61.271 (17)	O2^x—O3—O2^v	78.065 (15)
O3^v—O1—O3^iii	126.85 (2)	O2^x—O3—O2^ix	89.228 (15)
O3^v—O1—O3^viii	137.578 (15)	O2^x—O3—O3^v	74.547 (17)
O3^v—O1—O3^xiv	91.816 (13)

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