Rare earth niobate systems and the phosphor GdNbO4:Bi

An investigation to determine the compounds present in the Ln₂O₃Nb₂O₅ systems (Ln = La, Gd, and Y) and attempts to activate such compounds with Ti, In, Sb, Bi, Eu, and Tb revealed that only the orthoniobate (LnNbO₄) compound was a suitable host lattice and LnNbO₄:Bi under 2537 Å excitation produced the best phosphors. GdNbO₄:Bi is a brighter phosphor than the La and Y analogs, emitting at slightly higher energies. The position of the GdNbO₄:Bi peak emission at 4500 Å is independent of the activator concentration whereas peak emissions for LaNbO₄:Bi and YNbO₄:Bi move to lower energies with increasing Bi concentration.     

Uniform AMoO4:Ln (A=Sr, Ba; Ln=Eu, Tb) submicron particles: Solvothermal synthesis and luminescent properties

Rare-earth ions (Eu³⁺, Tb³⁺) doped AMoO₄ (A=Sr, Ba) particles with uniform morphologies were successfully prepared through a facile solvothermal process using ethylene glycol (EG) as protecting agent. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectra (XPS), Fourier transform infrared spectroscopy (FT-IR), photoluminescence (PL) spectra and the kinetic decays were performed to characterize these samples. The XRD results reveal that all the doped samples are of high purity and crystallinity and assigned to the tetragonal scheelite-type structure of the AMoO₄ phase. It has been shown that the as-synthesized SrMoO₄:Ln and BaMoO₄:Ln samples show respective uniform peanut-like and oval morphologies with narrow size distribution. The possible growth process of the AMoO₄:Ln has been investigated in detail. The EG/H₂O volume ratio, reaction temperature and time have obvious effect on the morphologies and sizes of the as-synthesized products. Upon excitation by ultraviolet radiation, the AMoO₄:Eu³⁺ phosphors show the characteristic ⁵D₀–⁷F_1–4 emission lines of Eu³⁺, while the AMoO₄:Tb³⁺ phosphors exhibit the characteristic ⁵D₄–⁷F_3–6 emission lines of Tb³⁺. These phosphors exhibit potential applications in the fields of fluorescent lamps and light emitting diodes (LEDs).     

Synthesis and magnetic properties of ALnO2 (A=Cu or Ag; Ln=rare earths) with the delafossite structure

Synthesis, structures, and magnetic properties of ternary rare earth oxides ALnO₂ (A=Cu or Ag; Ln=rare earths) have been investigated. CuLnO₂ (Ln=La, Pr, Nd, Sm, Eu) were synthesized by the direct solid state reaction of Cu₂O and Ln₂O₃, and AgLnO₂ (Ln=Tm, Yb, Lu) were obtained by the cation-exchange reaction of NaLnO₂ and AgNO₃ in a KNO₃ flux. These compounds crystallized in the delafossite-type structure with the rhombohedral 3R type (space group: R-3m). Magnetic susceptibility measurements showed that these compounds are paramagnetic down to 1.8 K. Specific heat measurements down to 0.4 K indicated that CuNdO₂ ordered antiferromagnetically at 0.8 K.     

Multiband orange-red photoluminescence of Eu ions in new “114” LnBaZn3GaO7 and LnBaZn3AlO7 oxides

A new series of gallozincates LnBaZn₃GaO₇ (Ln=La, Nd, Sm, Eu, Gd, Dy, Y) and new aluminozincates LnBaZn₃AlO₇ (Ln=Y, Eu, Dy) have been synthesized. Their structure refinements show that these phases belong to the “114” series, with hexagonal P6₃mc space group previously described for SmBaZn₃AlO₇. The photoluminescence study of these oxides shows that the Eu³⁺ activated LnBaZn₃MO₇ oxides with Ln=Y, La, Gd; and M=Al, Ga exhibit strong magnetic and electric dipole transitions (multiband emission) which is of interest for white light production. These results also confirm that the site occupied by Eu³⁺ is not strictly centrosymmetric. The electric dipole transition intensity is the highest in GdBaZn₃MO₇ [M=Al, Ga]: 0.05Eu³⁺ as compared with other Eu³⁺ activated compositions. This is due to the layer distortion around GdO₆ octahedra when compared with YO₆ and LaO₆ octahedra.     

Complex compounds of 2,2′-bipyridyl with some rare-earth bromides

The preparation of a series of new compoundsLnBr₃(2-bipy)₂·6H₂O (Ln=La, Pr, Nd, Eu, Gd) andLnBr₂OH(2-bipy)₂·4H₂O (Ln=Pr, Nd, Sm, Eu, Gd) is described. The IR spectra of these compounds are discussed. The thermal decomposition of compoundsLnBr₃(2-bipy)₂·6H₂O has been investigated.

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2,2-Bipyridylkomplexe einiger Seltenerdmetallbromide

Zusammenfassung Es wurden 2,2-Bipyridylkomplexe des TypsLnBr₃(2-bipy)₂·6H₂O (Ln=La, Pr, Nd, Sm, Eu, Gd) undLnBr₂OH(2-bipy)₂·4H₂O (Ln=Pr, Nd, Sm, Eu, Gd) dargestellt. Die IR-Spektren werden diskutiert. Die thermische Zersetzung von VerbindungenLnBr₃(2-bipy)₂·6H₂O wird untersucht.

     

Thermal decomposition of light lanthanide diglycolates

The conditions of thermal decomposition of La, Ce(III), Pr, Nd, Sm, Eu and Gd diglycolates have been studied. On heating, the diglycolates of Ce(III), Pr, Eu and Gd lose crystallization water and yield anhydrous salts, which are then transformed into oxides. The diglycolates of La, Nd and Sm are decomposed in three stages. First, the diglycolates undergo dehydration to form the anhydrous salts, which are next decomposed to Ln₂O₂CO₃. In the last step the thermal decomposition of Ln₂O₂CO₃ to Ln₂O₃ takes place, accompanied by an endothermic effect.     

Herstellung,Analyse und röntgenographische Identifizierung der Dioxymonocarbonate des Lanthans und der Lanthanidenelemente

Zusammenfassung Von anderen Autoren angestellte thermische Zersetzungsanalysen der hydratisierten Oxalate, Carbonate und wasserfr. Carbonate der Lanthanide zeigten, daß viele Lanthanidenelemente VerbindungenLn ₂O_2+x (CO₃)_1–x bilden. Für die Verbindungen des La und des Pr ist der Wert vonx nahezu Null; bei den anderen Elementen nimmt er mit wachsender Ordnungszahl zu.In diesen Arbeiten wurden hydratisierte Oxalate und, im Falle des Lanthans, das hydratisierte Carbonat unter verschiedensten Bedingungen zersetzt. Die resultierenden Festkörper wurden durch chemische und röntgenographische Verfahren analysiert.Es wurden drei polymorphe kristalline Formen vonLn ₂O₂CO₃ gefunden und charakterisiert. Der monokline TypIA tritt beiLn=La, Pr und Nd auf, der tetragonale TypI beiLn=La, Nd, Sm, Eu und Gd; der hexagonale TypII beiLn=La, Pr, Nd, Sm und Gd.

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Preparation, analysis and X-ray identification of the dioxymonocarbonates of lanthanum and the lanthanide elements

Thermal decomposition studies of hydrated lanthanide oxalates and carbonates or anhydrous carbonates, carried out by others, have revealed the formation ofLn ₂O_2+x (CO₃)_1–x for many of the lanthanide elements. The value ofx is very close to zero for the La and Pr compounds and it increases with atomic number for the heavier elements.In these studies, hydrated oxalates and in the case of lanthanum, the hydrated carbonate have been decomposed under a variety of conditions and the resulting solid materials examined by chemical analysis and by X-ray diffraction methods.Three polymorphic crystalline forms ofLn ₂O₂CO₃ are observed and characterized. Monoclinic TypeIA has been observed forLn=La, Pr and Nd; tetragonal TypeI forLn=La, Nd, Sm, Eu, and Gd; and hexagonal TypeII forLn=La, Pr, Nd, Sm, and Gd.

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Mit 5 Abbildungen

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Diese Arbeit wurde von der United States Atomic Energy Commission unterstützt. Menschen aus drei Kontinenten haben sie erarbeitet und wünschen sie Professor Dr.H. Nowotny zu widmen.     

Verbindungen der Seltenerdelemente mit α-Benzoinoxim, 1. Mitt.

Zusammenfassung Substanzen der ZusammensetzungLnCl₃·3H₂ Box ^* (Ln=La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Y) und LaBr₃·3 H₂ Box wurden isoliert und durch Thermoanalyse, IR-Absorptionsspektren und Röntgenstreuung charakterisiert.

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Compounds of the rare earth elements with -benzoin oxime

Compounds of compositionLnCl₃·3 H₂ Box ^* (Ln=La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Y) and LaBr₃·3H₂ Box were isolated and characterized by thermoanalysis, IR spectroscopy and X-ray diffraction.

     

Recent Research Progress of Red Phosphors for PDPs Applications

The commercial vacuum ultraviolet (VUV) red phosphor (Y, Gd)BO₃:Eu³⁺ has low luminous efficiency and poor color purity. Our work aims to overcome this drawback and we mainly devote to investigating the luminescence mechanism, improving the commercial red phosphor, and seeking for new red emitting VUV materials with high efficiency. Based on the investigation of the photoluminescence mechanism of VUV phosphors, both the luminous efficiency and the color purity of (Y, Gd)BO₃:Eu³⁺ are improved. Additionally, a series of novel VUV red phosphors have been developed, such as (Gd,Y)Al₃(BO₃)₄:Eu³⁺ and (La,Gd)P₃O₉:Eu³⁺. This presentation is a review about the recent research progress of red phosphors for plasma displays (PDPs) applications in our group.     

Facile sonochemical synthesis and photoluminescent properties of lanthanide orthophosphate nanoparticles

Uniform lanthanide orthophosphate LnPO₄ (Ln=La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho) nanoparticles have been systematically synthesized via a facile, fast, efficient ultrasonic irradiation of inorganic salt aqueous solution under ambient conditions without any surfactant or template. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), photoluminescence (PL) spectra as well as kinetic decays were employed to characterize the samples. The SEM and the TEM images show that the hexagonal structured lanthanide orthophosphate LnPO₄ (Ln=La, Ce, Pr, Nd, Sm, Eu, Gd) products have nanorod bundles morphology, while the tetragonal LnPO₄ (Ln=Tb, Dy, Ho) samples prepared under the same experimental conditions are composed of nanoparticles. HRTEM micrographs and SAED results prove that these nanostructures are polycrystalline in nature. The possible formation mechanism for LnPO₄ (Ln=La-Gd) nanorod bundles is proposed. Eu³⁺-doped LaPO₄ and Tb³⁺-doped CePO₄ samples were also prepared by using the same synthetic process, which exhibit an orange-red (Eu³⁺:⁵D₀-⁷F_{1, 2, 3, 4}) and green (Tb³⁺, ⁵D₄-⁷F_{3, 4, 5, 6}) emission, respectively.     

4,4′-Dipyridyl complexes of rare-earth thiocyanates

4,4-Dipyridyl complexes of rare-earth thiocyanates of the formulaLn(4-dipy)₂(NCS)₃·5H₂O (Ln = La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, Lu, Y, 4-dipy = 4,4-dipyridyl) have been synthesized. The IR spectra of these compounds and other physical properties are discussed. The thermal decomposition of some compounds (in the order Gd ÷ Lu) has been investigated.

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4,4-Dipyridylkomplexe von Seltenerdmetallthiocyanaten

Zusammenfassung Es wurden 4,4-Dipyridylkomplexe des TypesLn(4-dipy)₂(NCS)₃·5H₂O mitLn = La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, Lu und Y dargestellt. Die IR-Spektren und andere physikalische Eigenschaften werden diskutiert und die thermische Zersetzung von einigen Verbindungen (in der Reihe Gd ÷ Lu) untersucht.

     

Synthesis and luminescence studies on lanthanoid(III) complexes of aSchiff base derived from 2-acetylpyridine andtris-(2-aminoethyl)-amine

Summary A potentially heptadentated ligand (apytren) was obtained by condensation of 2-acetylpyridine andtris-(2-aminoethyl)-amine in the presence of lanthanoid(III) cations. Complexes of the formulaLn(apytren)(NO₃)₃·H₂O (Ln=La, Eu, Gd, and Tb) have been isolated and characterized, both in the solid state and in solution, by means of vibrational and electronic spectroscopy and of conductometric measurements. Their photophysical properties, including emission quantum yields and lifetimes, were studied and are discussed.

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Synthese und Lumineszenzuntersuchungen an Lanthanoid(III)-Komplexen mit einer aus 2-Acetylpyridin undtris-(2-Aminoethyl)-amin hergeleitetenSchiffschen Base

Zusammenfassung Durch Kondensation von 2-Acetylpyridin undtris-(2-Aminoethyl)-amin in der Gegenwart von Lanthanoid(III)-Kationen wurde ein potentiell siebenzähniger Ligand (apytren) erhalten. Komplexe der ZusammensetzungLn(apytren)(NO₃)₃·H₂O (Ln=La, Eu, Gd und Tb) wurden isoliert und sowohl im festen Zustand als auch in Lösung mittels IR-und UV-Vis-Spektroskopie und Leitfähigkeitsmessungen charakterisiert. Ihre photophysikalischen Eigenschaften, einschließlich Emissionsquantenausbeute und Lebensdauer, wurden untersucht und werden diskutiert.

     

Spectroscopic properties of Ln2MoO6:Eu3+

The spectroscopic properties of Ln₂MoO₆:Eu³⁺ (Ln = La, Gd, Y) compounds were investigated. The differences in the recorded fluorescence spectra are in accord with the different structures. For the La₂MoO₆:Eu³⁺ case, the spectrum is compatible with a C₂ point site symmetry. It appears that the energy level scheme is connected with the rare earth oxychloride one, so it is possible to determine accurately sets of crystal field parameters simulating the spectrum. For the other compounds, the Eu³⁺ ions occupy three different point sites. By using the site-selective excitation on the ⁵D₀ level it is possible to identify the energy level scheme characterizing each point site.     

4,4′-Dipyridyl and 2,2′-dipyridyl complexes of rare-earth perchlorates

4,4-Dipyridyl and 2,2-dipyridyl complexes of rare-earth perchlorates of the formulaLn(4-dipy)₈(ClO₄)₃HClO₄ · 4H₂O (Ln=La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, Lu, Y, 4-dipy=4,4-dipyridyl) andLn(2-dipy)₃(ClO₄)₃ · 6H₂O (Ln=Pr, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, Lu, Y, 2-dipy = 2,2-dipyridyl) have been synthesized. The IR spectra of these compounds and other physical properties are discussed.

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4,4-Dipyridyl- und 2,2-Dipyridylkomplexe von Seltenerdmetallperchloraten

Zusammenfassung Es wurden 4,4-Dipyridylkomplexe des TypsLn(4-dipy)₈(ClO₄)₃HClO₄ · · 4 H₂O mitLn=La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, Lu, Y und 2,2-Dipyridylkomplexe des TypsLn(2-dipy)₃(ClO₄)₃ · 6 H₂O mitLn=Pr, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, Lu und Y dargestellt. Die IR-Spektren und andere physikalische Eigenschaften werden diskutiert.

     

Syntheses and Structures of the Quaternary Copper Tellurides K3Ln4Cu5Te10 (Ln=Sm, Gd, Er), Rb3Ln4Cu5Te10 (Ln=Nd, Gd), and Cs3Gd4Cu5Te10

Six quaternary alkali-metal rare-earth copper tellurides K₃Ln₄Cu₅Te₁₀ (Ln=Sm, Gd, Er), Rb₃Ln₄Cu₅Te₁₀ (Ln=Nd, Gd), and Cs₃Gd₄Cu₅Te₁₀ have been synthesized at 1123 K with the use of reactive fluxes of alkali-metal halides ACl (A=K, Rb, Cs). All crystallographic data were collected at 153 K. These compounds crystallize in space group Pnnm of the orthorhombic system with two formula units in cells of dimensions (A₃Ln₄, a, b, c (Å)): K₃Sm₄, 16.590(2), 17.877(2), 4.3516(5); K₃Gd₄, 16.552(4), 17.767(4), 4.3294(9); K₃Er₄, 16.460(4), 17.550(4), 4.2926(9); Rb₃Nd₄, 17.356(1), 17.820(1), 4.3811(3); Rb₃Gd₄, 17.201(2), 17.586(2), 4.3429(6); Cs₃Gd₄, 17.512(1), 17.764(1), 4.3697(3). The corresponding R₁ indices for the refined structures are 0.0346, 0.0315, 0.0212, 0.0268, 0.0289, and 0.0411. The three K₃Ln₄Cu₅Te₁₀ structures belong to one structure type and the Rb₃Ln₄Cu₅Te₁₀ (Ln=Nd, Gd) and Cs₃Gd₄Cu₅Te₁₀ structures belong to another one, the difference being the location of one of the three unique Cu atoms. Both structure types are three-dimensional tunnel structures that contain similar Ln/Te fragments built from LnTe₆ octahedra and CuTe₄ tetrahedra. The CuTe₄ tetrahedra form ¹_∞[CuTe⁵⁻₃] and ¹_∞[CuTe³⁻₂] chains. The alkali-metal atoms, which are in the tunnels, are coordinated to seven or eight Te atoms.     

Syntheses, crystal structures and vibrational spectra of KLn(SO4)2·H2O (Ln=La, Nd, Sm, Eu, Gd, Dy)

The potassium lanthanide double sulphates KLn(SO₄)₂·H₂O (Ln=La, Nd, Sm, Eu, Gd, Dy) were obtained by evaporation of aqueous reaction mixtures of rare earth (III) sulphates and potassium thiocyanate at 298 K. X-ray single-crystal investigations show that KLn(SO₄)₂·H₂O (Ln=Nd, Sm, Eu, Gd, Dy) crystallise monoclinically (Ln=Sm: P2₁/c, Z=4, a=10.047(1), b=8.4555(1), c=10.349(1) Å, wR2=0.060, R1=0.024, 945 reflections, 125 parameters) while KLa(SO₄)₂·H₂O adopts space group P3₂21 (Z=3, a=7.1490(5), c=13.2439(12) Å, wR2=0.038, R1=0.017, 695 reflections, 65 parameters). The coordination environment of the lanthanide ions in KLn(SO₄)₂·H₂O is different in the case of the Nd/Sm/Gd and the Eu/Dy compounds, respectively. In the first case the Ln atoms are nine-fold coordinated in contrast to the latter where the Ln ions are eight-fold coordinated by oxygen atoms. The vibrational spectra of KLn(SO₄)₂·H₂O and the UV-vis reflection spectra of KEu(SO₄)₂·H₂O and KNd(SO₄)₂·H₂O are also reported.     

Rapid synthesis of ultrafine K2Ln2Ti3O10 (Ln=La, Nd, Sm, Gd, Dy) series and its photoactivity

Ultrafine-layered lanthanon titanates K₂Ln₂Ti₃O₁₀ (Ln=La, Nd, Sm, Gd, Dy) were fabricated at relatively low temperature by a stearic acid method (SAM). The obtained products were characterized by FT-IR, X-ray diffractometer, DTA-TG, scanning electron microscopy, transmission electron microscopy and BET experiments. The photocatalytic activity of the obtained products was studied and was compared with that of solid-state reaction (SSR) using photodecomposition of methyl orange as the model system. Results showed that by using SAM, the fabricating temperature was lowered (from 1100 to 800 °C) and the reacting time was shortened (from at least 11-2 h). Comparing with the product of traditional SSR, the particle size of K₂Ln₂Ti₃O₁₀ synthesized by SAM is smaller, BET surface area is higher (more than 16.97 m²/g), and photoreactivity is better. It was very interesting to find the difference in d(002) of obtained K₂Ln₂Ti₃O₁₀ for Ln=La, Nd, Sm, Gd, Dy separately and the photoactivity of K₂Ln₂Ti₃O₁₀ is strongly dependent on lanthanide, increasing in the sequence of La<Sm<Nd<Gd <Dy. A possible reason was put forward.     

Triphenylphosphane Oxide Complexes of Lanthanide Nitrates: Polymorphs and Photophysics

A series of mer‐[Ln(NO₃)₃(Ph₃PO)₃] complexes were prepared from Ln(NO₃)₃ · xH₂O and Ph₃PO in chloroform (Ln = La, Nd, Sm, Eu, Gd, Tb, Dy, and Er). The La and Nd complexes were 0.25 CHCl₃ solvates, whereas the others were solvent‐free. The identical reaction using Yb(NO₃)₃ · xH₂O produced the unique salt trans‐[Yb(NO₃)₂(Ph₃PO)₄][Yb(NO₃)₄(Ph₃PO)] · Et₂O. All nitrate ions in all complexes are η²‐chelating. A comparison of the various [Ln(NO₃)₃(Ph₃PO)₃] structures, including those in the literature, reveals at least four common polymorphs, each of which is represented by isomorphic structures of multiple Ln ions. Luminescence of mer‐[Ln(NO₃)₃(Ph₃PO)₃] (Ln = Y, La, Nd, Sm, Eu, Gd, Tb, and Dy), trans‐[Yb(NO₃)₂(Ph₃PO)₄][Yb(NO₃)₄(Ph₃PO)] and Ph₃PO assignments are reported. Latva's empirical rule allows for the antenna effect, in which energy is transferred from the triplet state of the Ph₃PO ligand, to occur only for Tb³⁺. Excitation via Ph₃PO results in strong green luminescence for Tb³⁺ having twice the intensity as that which results from direct excitation of the f‐f transitions.     

Mono and trinuclear lanthanide complexes of 13-membered tetraaza macrocycle: Synthesis and characterization

The reaction of 1,8-diamino-3,6-diazaoctane and diethyl malonate in dry methanol yielded a 13-membered macrocycle. Complexes of the type [Ln(tatd)Cl₂ (H₂O)₃]Cl [Ln^III=La, Pr, Nd, Sm, Eu, Gd, Tb, Dy; tatd=1, 5, 8, 11-tetra-azacyclotridecane-2,4-dione] have been synthesized by template condensation. The complex [La(tatd)Cl₂ (H₂O)₃]Cl in methanol was reacted with lanthanide chlorides to yield the trinuclear complexes of type [2{La(tatd)Cl₂(H₂O)₃}LnCl₃]Cl₂ [Ln^III=La, Pr, Nd, Sm, Eu, Gd, Tb, Dy]. The chemical compositions of mono and trinuclear complexes have been established on the basis of analytical, molar conductance, electrospray (ES) and fast atom bombardment (FAB) mass data. In mononuclear complexes the Ln³⁺ ion is encapsulated by four ring nitrogens and in trimetallic complexes the exo-carbonyl oxygens of two mononuclear units coordinate to the Ln³⁺ ions resulting in a polyhedron around the lanthanide ions. Thus the macrocycle is bonded in a tetradentate fashion in the former complexes and hexadentate in the latter. The coordination number nine around the encapsulated Ln³⁺ and seven around the exo-oxygen bonded Ln³⁺ ions are established. The symmetry of the ligand field around the metal ions is indicated from the emission spectra.     

New insight into the symmetry and the structure of the double perovskites Ba2LnNbO6 (Ln=lanthanides and Y)

Structures of the double perovskites Ba₂LnNbO₆ (Ln=La, Pr, Nd, Sm, Eu, Tb, Dy, Ho, and Y) at room temperature have been re-examined by Rietveld profile analysis of X-ray diffraction data. It was shown that the correct phase sequence across the lanthanides is I2/m (Ln=La, Pr, Nd, and Sm), I4/m (Ln=Eu, Gd, Tb, and Dy), and (Ln=Ho and Y), respectively. All phases can be derived from the ideal cubic perovskite by ordering the Ln(III) and Nb(V) ions and by out-of-phase tilting the LnO₆/NbO₆ octahedra around either the primitive two-fold [110]_p-axis (I2/m) or the four-fold [001]_p-axis (I4/m). The monoclinic P2₁/n structure that contains both out-of-phase and in-phase tilt around the primitive [110]_p- and [001]_p-axis, respectively, has not been observed for this series of compounds.