Über Oxide mit dem “Butterfly-Motiv”: Rb6[Fe2O5] und K6[Fe2O5]

New Oxides with the “Butterfly-Motive”: Rb₆[Fe₂O₅] and K₆[Fe₂O₅] Rb₆[Fe₂O₅] and K₆[Fe₂O₅] were obtained for the first time by annealing intimate mixtures of “Rb₆CdO₄” with CdO (molar ratio 1 : 1.1) and KO_0.48 with CdO (molar ratio 5.9 : 1) respectively in closed Fe-cylinders. Determination and refinement of the crystalstructure confirms the space group C2/m (four-circle-diffractometer data). Rb₆[Fe₂O₅]: Ag Kα , 720 out of 1220 Io(hkl), R = 9.68%, R_w = 6.09%; a = 718.9pm, b = 1183.1 pm, c = 695.4pm, β = 95.05°, Z = 2; K₆[Fe₂O₅]: MoKα , 1214 Out of 12141_o(hkl), R = 3.20070, R_w = 2.48%, a = 691.21 pm, b = 1142.78pm, c = 665.50pm, β = 93.82°, Z = 2. The binuclear unit [O₂FeOFeO₂]^6? already known to be planar with oxoferrates(II) now was observed to be angular here and closely related to Na₆[Be₂O₅].     

Über ein neues Oxoferrat mit „Butterfly-Motiv”︁: K2Na4[Fe2O5]

A New Oxoferrate with “Butterfly-Motiv”: K₂Na₄[Fe₂O₅] Dark red-brown single-crystals of K₂Na₄[Fe₂O₅] were obtained for the first time by heating “K₃Na₃CdO₄” at 500°C in closed Fe-cylinders. Determination and refinement of the crystal structure confirms the space group P4₂/mnm (No. 136). Four-circle diffractometer data: MoKα , 373 out of 373 I_o(hkl); R = 5.3%; R_w = 4.6%; a = 645.94(5), c = 1 039.2(1) pm. In contrast to the already known oxoferrates(II) with the “Butterfly-Motiv”, Rb₆[Fe₂O₅] and K₆[Fe₂O₅] [1], we now found an isotypic structure for K₂Na₄[Fe₂O₅] with the oxocobaltates of Rb₂Na₄[Co₂O₅] and K₂Na₄[Co₂O₅] [2].     

Das erste Oxogermanat mit „Stuffed Pyrgoms”︁: CsNa3Li8{Li[GeO4]}4

The First Oxogermanate with “Stuffed Pyrgoms”: CsNa₃Li₈{Li[GeO₄]}₄ By heating the binary oxides CsO_0.52, NaO_0.45, LiO_0.5 and GeO₂ in the mol. ratio 1.24:1.4:6.7:3.9 (Ni tubes; 600°C/89 resp. 65 d) we obtained for the first time CsNa₃Li₈{Li[GeO₄]}₄ in form of pale yellow prisms as well as powder: space group I4/m (I. T. No. 87) with a = 1 120.73(5); c = 651.64(5) pm; Z = 2; (four circle diffractometer data; MoKα; 5 152 I_o(hkl); R = 1.7%; R_w = 1.6%), parameters see text. The structure determination confirmes its being isotypic with CsKNaLi₉{Li[SiO₄]}₄, CsKNa₂Li₈{Li[SiO₄]}₄ and RbNa₃Li₈{Li[SiO₄]}₄. The Madelung part of lattice energy (MAPLE), effective coordination numbers (ECoN), mean fictive ionic radii (MEFIR) and the charge distribution (CHARDI) are calculated.     

Ketten aus Ringen: K5{Li[Ge2O7]} — das erste „Litho-Digermanat”︁

Chains consisting of Rings: K₅{Li[Ge₂O₇]} — the First ‘Litho-Digermanate’ By heating of a well-ground mixture of the binary oxides KO_0.55, Li₂O and GeO₂ (K: Li: Ge = 6.1 : 2.2 : 2; Ni-tube; 600°C; 49 d) we obtained for the first time single crystals of K₅{Li[Ge₂O₇]}. This ‘lithodigermanate’ represents a completely new type of structure: monoclinic, space group P2₁/c, a = 624.9(2) pm, b = 1586.6(8) pm; c = 1058.3(6) pm and β = 109.38(4)°; Guinier-Simon data, Z = 4. The structure was solved by four-circle diffractometer data [Siemens AED II, Mo? Kα ; 2872 I_o(hkl); R = 4.5%, R_w = 3.3%], parameters see text. The Madelung Part of Lattice Energy, MAPLE, and Effective Coordination Numbers, ECoN, these calculated via Mean Fictive Ionic Radii, MEFIR, as well as charge distribution CHARDI, are calculated and discussed.     

Neue Silicate mit „Stuffed Pyrgoms”︁: CsKNaLi9{Li[SiO4]}4, CsKNa2Li8{Li[SiO4]}4, RbNa3Li8{Li[SiO4]}4 [1] und RbNaLi4{Li[SiO4]}2 [2]

More Silicates with ?Stuffed Pyrgoms”?: CsKNaLi₉{Li[SiO₄]}₄, CsKNa₂Li₈{Li[SiO₄]}₄, RbNa₃Li₈{Li[SiO₄]}₄ [1] and RbNaLi₄{Li[SiO₄]}₂ [2] Single crystals of the new silicates CsKNaLi₉{Li[SiO₄]}₄, CsKNa₂Li₈{Li[SiO₄]}₄, RbNa₃Li₈{Li[SiO₄]}₄ and RbNaLi₄{Li[SiO₄]}₂ as well as powder (Rb-containing compounds only) were obtained for the first time. The samples were prepared by heating well ground mixtures of the binary oxides in Ni and Ag tubes, respectively. The structure determination was carried out by four-circle diffractometer data (MoKα radiation; Siemens AED 2): CsKNaLi₉{Li[SiO₄]}₄: tetragonally prismatic crystals, light yellow; 726 I₀(hkl), R = 4.4%, R_w = 2.8%; a = 1 102.0(6), c = 637.9(5) pm; Z = 2; space group I4/m; 2 CsO_0.55 + Li₄TlO₄ + glas (560°C, 15 d). CsKNa₂Li₈{Li[SiO₄]}₄: tetragonally prismatic crystals, light yellow; 727 I₀(hkl), R = 4.4%, R_w = 2.6%; a = 1 103.5(7), c = 637.7(4) pm; Z = 2; space group I4/m; 1.1 CsO_0.61 + 1.1 KO_0.55 + 1.4 NaO_0.52 + 6.5 Li₂O + 4 SiO₂ (600°C, 60 d). RbNa₃Li₈{Li[SiO₄]}₄: tetragonally prismatic crystals, colourless; 600 I₀(hkl), R = 2.3%, R_w = 2.0%; a = 1 092.08(6), c = 632.76(4) pm; Z = 2; space group I4/m; 4 RbO_0.57 + 3 NaO_0.52 + 6.5 Li₂O + 4 SiO₂ (650°C, 63 d). RbNaLi₄{Li[SiO₄]}₂: monoclinic, ball-shaped, colourless; 1 224 I₀(hkl), R = 3.1%, R_w = 3.1%; a = 1 573.10(13), b = 630.48(5), c = 781.25(8) pm, b = 90.566(8)°; Z = 4; space group C2/m; 1.1 RbO_0.52 + 1.2 NaO_0.45 + 5 Li₂O + 4 SiO₂ (700°C, 40 d).     

Zur Kenntnis von Unordnungsphänomenen bei Oxiden mit dem „Schmetterlingsmotiv”︁ Über Rb2K4[O2CoOCoO2]

On the Knowledge of Disorder Phenomena in Oxides with the Motive of Butterfly. On Rb₂K₄[O₂CoOCoO₂] For the first time Rb₂K₄Co₂O₅ was obtained by annealing intimate mixtures of the binary oxides as dark red single crystals (Rb:K:Co = 1.2:3.2:1; Ni-tube; 570°C; 20 d). Structure Refinement [fourcircle diffractometer data; PW 1100; AgK radiation; 401 of 607 I₀(hkl); R = 9.9%; R_w = 5.5%; space group P4₂/mnm; Z = 2; a = 674,2(4), c = 1172,2(6) pm] confirms the isotypism to K₂Na₄Co₂O₅, Rb₂Na₄Co₂O₅ [2] and K₂Na₄Be₂O₅ [3]. The Madelung Part of Lattice Energy, MAPLE, Effektive Coordination Numbers, ECoN, these via Mean Fictive Ionic Radii, MEFIR, and the Charge Distribution, ΣQ, were calculated. The isotypism of Rb₂K₄Co₂O₅ and Rb₂Na₄Co₂O₅ is compared graphically.     

Eine ‚Reduktive Austausch-Reaktion’︁: Einkristalle von Rb2[MnO4] aus Li[MnO4]

A ‘Reductive Exchange Reaction’: Single Crystals of Rb₂[MnO₄] from Li[MnO₄] By heating of well ground mixtures of the oxides RbO_0.9 and Li[MnO₄] (Rb:Mn = 1.5:1; Ag-tubes; 660°C, 56 d) dark-green, orthorhombic single-crystals of Rb₂[MnO₄] were obtained. The structural determination revealed the isotype to K₂[MnO₄]; a β-K₂SO₄-typ. Thus, for the first time by a pure solid-state-chemical way, via a reductive, complete alkali-metal exchange, single crystals of an oxomanganate(VI) of the alkali metals were prepared. The Madelung Part of the Lattice Energy, MAPLE, and the charge distribution were calculated.     

Über „Orthoindate”︁ der Alkalimetalle: Zur Kenntnis von K5[InO4]

A New ?Orthoindate”? of an Alkali Metal: K₅[InO₄] Hitherto unknown K₅[InO₄] was prepared by heating intimate mixtures of K₂O, In₂O₃ and elementar In (molar ratio 10.0 : 1.0 : 4.0) in closed Ni-cylinders (30 days, 500°C) in form of pale red, nearly colourless, transparent, single crystals. Same crystals were obtained by heating mixtures of K₂O, CdO and elementar In (molar ratio 3.1 : 1.0 : 1.0) in closed Ag-cylinders (30 days, 450°C), too. In this case we also found yellow-brown crystals of K₁₄[In₄O₁₃] [1]. Structure determination by four circle diffractometer data (MoKα, 15279 out 17454 I_o(hkl), R = 5.60%, R_w = 5.25%). Space group P1 with a = 1827.9 pm; b = 1694.4 pm; c = 1329.4 pm; α = 113.3°; β = 111.4°; γ = 105.2°; Z = 16. Characteristic feature of the structure are isolated [InO₄]^5?-tetraeder. The Madelung Part of Lattice Energy, MAPLE, the Mean Fictive Ionic Radii, MEFIR, Effective Coordination Numbers, ECoN, and Charge Distribution, VADI, are calculated.     

Das erste Titanat mit „Stuffed Pyrgoms”︁: RbNa3Li12[TiO4]4 = RbNa3Li8{Li[TiO4]}4

The First Titanate with ?Stuffed Pyrgoms”?: RbNa₃Li₁₂[TiO₄]₄ = RbNa₃Li₈{Li[TiO₄]}₄ By heating a well grounded mixture of the binary oxides Rb₂O, Na₂O, Li₂O, and TiO₂ [Rb:Na:Li:Ti = 1.1:3.1:12.5:4.0; 780°C, 41 d] we obtained RbNa₃Li₈{Li[TiO₄]}₄ as colourless platelike crystals. This first titanate with ?stuffed pyrgoms”? is isostructural with RbNa₃Li₈{Li[SiO₄]}₄, CsKNa₂Li₈{Li[SiO₄]}₄ and CsKNaLi₉{Li[SiO₄]}₄ [2]. The compound crystallizes tetragonal I4/m with a = 1 125.8(1) pm and c = 652.4(1) pm (Guinier-Simon-Data, Z = 2). The structure was determined by four-cyrcle-data (Siemens AED2, MoK) and leds to the residual values R = 3.7% and R_w = 3.1% (additional data see text). The Madelung Part of Lattice Energy (MAPLE), Effective Coordination Numbers (ECoN), Mean Fictive Ionic Raddii (MEFIR) and the Charge Distribution in Solids (CHARDI) are calculated and discussed.     

Das erste „Litho-Manganat(V)”︁ mit Schichtstruktur: Cs2{Li[MnO4]}

The First “Litho-Manganate(V)” with Layer-Structure: Cs₂{Li[MnO₄]} By heating intimate mixtures of the oxides [CsO_1,2, Li₂MnO₃; Cs: Mn = 2,3 : 1; Ag-Zylinder, 580°C, 62 d] blue-green single crystals of Cs₂{Li[VO₄]} were obtained for the first time. The new “Litho-Manganate(V)” crystallices orthorhombic (SG: Cmc2₁) with a = 596.08(7), b = 1202.6(1), c = 816.8(1) pm (Guinier-Simon data), Z = 4. It is isotypic with Cs₂{Li[VO₄]} [1]. The structure was determined by four-circle-diffractometer data [Mo? Kα , for 496I_o(hkl) R = 3.1%, R, = 2.4%], parameters see text. The Madelung Part of Lattice Energie, MAPLE and Effective Coordination Numbers, ECoN, these calculated via Mean Fictive Ionic Radii, MEFIR, are calculated and disscussed.     

Darstellung und Kristallstruktur von K6[Al2O6] und Rb6[Al2O6]

Preparation of Crystal Structure of K₆[Al₂O₆] and Rb₆[Al₂O₆] Colourless single crystals of K₆[Al₂O₆] have been prepared from intimate mixtures of KAlO₂ and K₂O (550°C, 90 d). The structure determination from four-circle diffractometer data (MoKα , 742 I_o(hkl), R = 2.2%, R_w = 2.1%) confirms the space group C2/m with Z = 2; a = 698.25 pm, b = 1 103.54 pm, c = 646.49 pm, β = 102.49°. Colourless single crystals of hitherto unknown Rb₆[Al₂O₆] have been prepared from intimate mixtures of RbAlO₂ and Rb₂O (520°C, 120 d). The structure determination from four-circle diffractometer data (MoKα , 1 240 I_o(hkl)) results in the residual values R = 7.2%, R_w = 4.9%; space group C2/m; a = 725.92 pm, b = 1 143.33 pm, c = 678.06 pm, β = 104.05°; Z = 2. K₆[Al₂O₆] and Rb₆[Al₂O₆] are isostructural with K₆[Fe₂O₆]. A characteristic structure unit is the anion [Al₂O₆]^6? consisting of two edge-sharing [AlO₄] tetrahedra. Effective Coordination Numbers (ECoN), Mean Fictive Ionic Radii (MEFIR), the Madelung Part of Lattice Energy (MAPLE) and the Charge Distribution (CHARDI) are calculated and discussed.     

Über „Gemischt-Koordinierte”︁ einkernige Anionen. 2. Ein Oxoruthenat(VI) neuen Typs: CsK5Ru2O9 = CsK5[RuO5][RuO4]

On ?Mixed-coordinated”? isolated Anions. 2. A New Type of Oxoruthenates(VI): CsK₅Ru₂O₉ = CsK₅[RuO₅][RuO₄] By heating of intimate mixtures of the oxides [KO_1.1, CsO_1.2, RuO₂; K:Cs:Ru = 2.2:1:1; Ag-tube, 750°C, 34d] dark-green single crystals of CsK₅Ru₂O₉ = CsK₅[RuO₅][RuO₄] were obtained for the first time. Ru⁶⁺ shows two different polyhedra in this oxide (tetrahedron, trigonal bipyramid). Untill now Rb₆[TeO₅][TeO₄] [1] next to ?Mg₄GeO₆”? [3] and the isotypic high-pressure silicate [4] was the only examples known. The structure was determined by four-circle diffractometer data [Mo? Kα , 1824 from 1949 I₀(hkl), SG: Pnma, R equals; 12.6%, R_w = 4.5%], parameters see text. Guinier-Simon data gave a = 1701.6(3), b = 827.6(1), c = 905.7(1) pm, Z = 4. The Madelung Part of Lattice Energy, MAPLE, Effective Coordination Numbers, ECoN, these calculated via Mean Fictive Ionic Radii, MEFIR, were calculated and discussed.     

Zur Kenntnis der Oxoplatinate Na2PtO2, Na2PtO3, „K2PtO3”︁ und „Rb2PtO3”︁

The oxoplatinates Na₂PtO₂, Na₂PtO₃, ?K₂PtO₃”? and ?Rb₂PtO₃”?. Hitherto unknown Na₂PtO₂ (greyish black) was prepared. Na₂PtO₂ (orthorhombic, D—Immm; a = 4.58₅, b = 3.11₉, c = 9.58₈ Å) is isotypic with Li₂CuO₂. α-Na₂PtO₃ (darkyellow; red as single-crystals) is monoclinic, C—C2/c (a = 5.41₉, b = 9.38₅, c = 10.75₂ Å, β = 99.67°), Li₂SnO₃-type. According to 3-dimensional single crystal data hitherto unknown β-Na₂PtO₃ (red crystals) is an orthorhombic variant of the Li₂SnO₃-type (a = 18.83₈, b = 6.28₂, c = 9.06₂ Å, Z = 16, D—Fddd; parameters see text); R = 0.080₉, R' = 0.094₈ [256 reflexes (hk0—hk6)]. The Madelung part of the lattice energy (MAPLE) is calculated and discussed for α-, β-Na₂PtO₃, α- and β-PtO₂. For the first time we got K₂PtO₃ and Rb₂PtO₃.     

Über „Lithovanadate”︁: Zur Kenntnis von Rb2[LiVO4] und Cs2[LiVO4]

On ?Lithovanadates”?: Rb₂[LiVO₄] and Cs₂[LiVO₄] By heating of well ground mixtures of the binary oxides [A₂O, Li₂O, V₂O₅, A : Li: V = 2.2 : 1.1 : 1.0 (A = Rb, Cs); Ni-tube, 750° 25 d] we obtained Rb₂[LiVO₄] and Cs₂[LiVO₄] colourless, orthorhombic single crystals. We found a new type of ?Lithovanadate”?-structure: space group Cmc2₁; a = 587.9(1), b = 1170.1(1), c = 793.3(1) pm, Z = 4 (A = Rb) bzw. a = 610.5(1), b = 1222.6(3), c = 815.5(2) pm, Z = 4 (A = Cs). The structure was determined by four-circle diffractometer data [MoKα -radiation; 997 from 1157 I₀(hkl), R = 7.75%, R_w = 5.54% (A = Rb); 686 from 686 I₀(hkl), R = 6.97%, R_w = 4.20% (A = Cs)] parameters see text. The Madelung part of Lattice Energy, MAPLE, and Effective Coordination Numbers, ECoN, these via Mean Fictive Ionic Radii, MEFIR, have been calculated.     

Li10Si2PbIIO10 = Li20[(SiO4)4(OPbO2PbO)] — Das erste „gemischte”︁ Silicat-Plumbat(II)

Li₁₀Si₂Pb^IIO₁₀ = Li₂₀[(SiO₄)₄(OPbO₂PbO)] — The first ?mixed”? Silicate-Plumbate(II) Colourless crystals of Li₁₀Si₂PbO₁₀ were obtained by heating a well-ground mixture of LiPb, Li₂O₂ and ?SiO₂”? (deriving from Duran glas) in Ag-tubes (650°C; 60 d). The crystal structure was determined (four-circle diffractometer data, Mo? K, 1 474 I_o(hkl), R = 4.2%, R_w = 2.8%, parameters see text). The silicate-plumbate crystallizes monoclinic (space group C2/m; I. T. No. 12) with a = 2985.1(4); b = 610.6(6); c = 512.8(1) pm, β = 99.70(9)° (four-circle data), Z = 4. Further the Madelung Part of Lattice Energy (MAPLE), Effective Coordination Numbers (ECoN), the Mean Fictive Ionic Radii (MEFIR) and the Charge Distribution (CHARDI) are being calculated.     

Synthese und Kristallstrukturen der Polyselenidokomplexe (PPh4)6[M(Se4)2]2[WSe4] · DMF mit M = Zink und Quecksilber

Syntheses and Crystal Structures of the Polyselenido Complexes (PPh₄)₆[M(Se₄)₂]₂[WSe₄] · DMF with M = Zinc and Mercury The title compounds have been prepared by the reactions of the acetates of zinc and mercury, respectively, with excess (PPh₄)₂ WSe₄ in boiling dimethylformamide, forming black-red single crystals. According to the X-ray structure determinations both compounds crystallize isotypically in the space group 12/a with four formula units per unit cell. (PPh₄)₆[Zn(Se₄)₂]₂[WSe₄] · DMF: a = 2888.1(6), b = 1740.3(2), c = 2893.9(4) pm, β = 90.47(1)°. 3230 observed unique reflections, R = 0.009. (PPh₄)₆[Hg(Se₄)₂]₂[WSe₄] · DMF: a = 2891.8(5), b = 1738.0(4), c = 2920.1(5) pm, β = 90.29(2)°. 2978 observed unique reflections, R = 0.115%. The compounds consist of PPh₄⁺ ions, spirocyclic octaseleno metallates [M(Se₄)₂]^2?, tetrahedral WSe₄²-ions, and disordered DMF Molecules.     

CuTb[B5O10]: Das erste „Metaborat”︁ mit einem [B5O10]5− -Anion

CuTb[B₅O₁₀]: The first “Metaborate” with a [B₅O₁₀]^5? Anion Single crystals of the new compound CuTb[B₅O₁₀] were obtained by a B₂O₃ flux-technique. They crystallize in a so far unknown structure. X-ray investigations on single crystals led to the space group C–I ba2 (Nr. 45); a = 6.294(1) Å; b = 8.406(8) Å; c = 12.733(2) Å; Z = 4. The structure contains [B₅O₁₀]^5? chains isolated from each other. These chains include twelf membered rings of boron and oxygen. Each ring consists of two tetrahedral BO₄ and two planar B₂O₅ groups and is connected with the next one via the BO₄ units. Tb³⁺ is eightfold- and Cu²⁺ elongated octahedraly coordinated by oxygen.     

Zur Oxydation intermetallischer Phasen: Die Oxoplumbate(II) K6[Pb2O5] [1] und K4[PbO3] [2]

On the Oxidation of Intermetallic Phases: The Oxoplumbates(II) K₆[Pb₂O₅] [1] and K₄[PbO₃] [2] Very pale yellow crystals of K₆[Pb₂O₅] were obtained by heating a wellground mixture of LiPb und K₂O₂ (K₂O₂: LiPb = 2.5:1) in Ag-tubes (550°C; 40 d). The crystal structure, triclinic, space group P1 , a = 1 326.7(6); b = 758.8(4); c = 637.0(3) pm; α = 92.17(3)°; β = 94.41(3)°; γ = 112.85(4)°; Z = 2 was determined (four-circle diffractometer data, Mo? K, 3 270 I_o(hkl), R = 8.0%, R_w = 3.5%, parameters see text). The pale yellow crystals of K₄[PbO3] were received by heating KPb and K₂O₂ (K₂O₂: KPb = 3.3:2) in Ni-tubes (450°C; 17 d). The crystal structure (orthorhombic, space group Pbca with a = 658.2(1); b = 1 131.8(4); c = 1 872.2(6) pm; Z = 8) was refined (four-circle diffractometer data, Mo? K, 2 003 I_o(hkl), R = 4.9%, R_w = 2.8%). The Madelung Part of Lattice Energy (MAPLE), Effective Coordination Numbers (ECoN), the Mean Fictive Ionic Radii (MEFIR) and the Charge Distribution (CHARDI) are being calculated for both oxides.     

Ein sauerstoffreiches Neodymcuprat: Hochdrucksynthese und Kristallstruktur von Nd12Cu6O25 („Nd2CuO4,17”︁)

An Oxygen-rich Neodymium Cuprate: High-pressure Synthesis and Crystal Structure of Nd₁₂Cu₆O₂₅ (“Nd₂CuO_4.17”) Single crystals of Nd₁₂Cu₆O₂₅ (“Nd₂CuO_4.17”) could be obtained by reaction of Nd₂O₃ und CuO with KO₂ in a modified Belt-type apparatus at 40 kbar, 1500°C. The compound crystallizes in the monoclinic space group C2/m with a = 17.128(6), b = 3.7288(6), c = 18.364(6) Å, β = 111.22(1)º and Z = 2. The structure refinement converged at R1 = 0.0302 for 1451 reflections with F_o > 4σ(F_o) and R1 = 0.0903, wR2 = 0.0767 for all 2571 data. The structure-determining feature are Cu(1,2)? O octahedral chains, interrupted by Cu(3) atoms in square-planar coordination. Each of this CuO₄ groups is connected to another one, resulting in short copper-copper distances (d_{Cu? Cu} = 3.012 Å). A comparison of the structure of “Nd₂CuO_4.17” with those of La₂CuO₄ and Nd₂CuO₄ makes the structural relations of the former with La₂CuO₄ and, hence, the aristotype K₂NiF₄ evident, in spite of the close stoichiometric similarity between Nd₂CuO₄ and “Nd₂CuO_4.17”.