Single Crystals of Cubic Rare-Earth Pyrochlore Germanates

Communication pubs.acs.org/IC

Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Single Crystals of Cubic Rare-Earth Pyrochlore Germanates: RE2Ge2O7 (RE = Yb and Lu) Grown by a High-Temperature Hydrothermal Technique Liurukara D. Sanjeewa,†,‡ Kate A. Ross,§,∥ Colin L. Sarkis,§ Harikrishnan S. Nair,§ Colin D. McMillen,† and Joseph W. Kolis*,†

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†

Department of Chemistry and Center for Optical Materials Science and Engineering Technologies, Clemson University, Clemson, South Carolina 29634-0973, United States ‡ Materials Science and Technology Division and Quantum Condensed Matter Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States § Colorado State University, Fort Collins, Colorado 80523, United States ∥ Quantum Materials Program, Canadian Institute for Advanced Research (CIFAR), Toronto, Ontario M5G 1Z8, Canada S Supporting Information *

ABSTRACT: Large single crystals of Yb2Ge2O7 in the cubic Fd3̅m space group, are synthesized and characterized from a high-temperature hydrothermal method (650°C/200 MPa in 1 M KF). The cubic phase displays spin frustration and possibly nonclassical quantum-spin behavior at low temperature. This is the first report of single crystals of this important phase of size and quality suitable for single-crystal neutron diffraction.

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yrochlores, with the generic formula A2B2O7 and a complex cubic structure (Figure 1), are important examples of 3D spin-frustrated materials.1−9 Both lattice sites have crystallographic 3(−)m symmetry but with different crystal fields. The A site, typically occupied by trivalent rare-earth (RE) ions, is an eight-coordinate compressed rhomb, while the B site is a distorted octahedron. The most magnetically interesting structural feature of the pyrochlores is the relationship between the various A and B sites within the unit cell. Both sites generate symmetry-defined corner-shared tetrahedra of metal ions with 3fold rotation axes. In particular, RE2B2O7 (B = Ti4+, Sn4+, or Ge4+) displays extremely unusual spin frustration, leading to complex magnetic behavior such as “spin ices”.10−13 Spin ices are the magnetic analogues of the structural frustration occurring in ordinary ice. Pauling originally pointed out that the structural requirements of two covalent O−H bonds and two hydrogen bonds around each oxygen atom are at odds with the 3-fold crystallographic site symmetry in the ice lattice.14 The analogous behavior in magnetic systems was first pointed out by Anderson.15 In the pyrochlore structure, RE metal ions are magnetically coupled through oxo bridges, creating two “spinin” and two “spin-out” magnetic vectors on the metal tetrahedra. Because the tetrahedra are located on 3-fold structural sites,16,17 this frustration leads to the massive degeneracy of ground states that can have a residual entropy even at 0 K.18 One way that the material may resolve this residual entropy is through strong quantum fluctuations, which lead to an exotic state called quantum spin ice,19−21 which is an example of a quantum spin © XXXX American Chemical Society

Figure 1. Structures of rare-earth germanate pyrochlores. Clockwise from upper left: (a) extended pyrochlore framework; (b) A-site REO8 compressed cube; (c) B-site GeO6 octahedron; (d) tetrahedral interactions of RE sites; (e) tetrahedral interactions of germanium sites.

liquid (QSL).22−24 QSLs were previously proposed in 1D chains and 2D Kagome and triangular nets.25−27 3D QSL candidates are extremely rare, however, so the pyrochlores present a unique opportunity for the observation of new forms of matter. Of the RE2B2O7 pyrochlores, titanates are most common because of their stability and ease of crystal growth.28 Stannates recently garnered attention for their ability to form spin ices (RE = Dy3+ and Ho3+) and other unusual phases.8,12,29−31 Extending this work to the smaller germanates is of interest because of the considerable sensitivity of the magnetic behavior to chemical Received: June 29, 2018

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DOI: 10.1021/acs.inorgchem.8b01810 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry pressure.32,33 In classical spin ices like Dy2Ti2O7 and Ho2Ti2O7, the magnetic exchange coupling constant (J) and the dipole− dipole exchange interaction (D) are functions of the unit cell size and orbital overlap, and because of this competition, smaller unit cells can lead to enhanced densities of the exotic magnetic monopole excitations for which spin ices are known.33 In ordered rare-earth pyrochlores with smaller moment sizes such as Yb2Ti2O7 and Er2Ti2O7, four symmetry-allowed anisotropic nearest-neighbor interactions compete to form a complex phase diagram that hosts several distinct magnetic orders as well as regions of strong quantum fluctuations (candidate regions for QSLs).31 Substituting different B-site cations is expected to change the relative weight of the four nearest-neighbor anisotropic exchange interactions, pushing the materials closer to the long-sought QSL regimes.21,30 The critical parameter of the pyrochlore phase stability is the ionic radius ratio between A3+ and B4+.6,34 When rA3+/rB4+ is greater than approximately 1.83, the cubic pyrochlore structure is destabilized in favor of lower-symmetry phases. In the case of Ti4+, the smaller RE ions Sm−Yb result in ratios smaller than 1.83, leading to cubic pyrochlore phases of RE2Ti2O7.28 For the larger ions La−Nd, the stable structure is a polar monoclinic phase in space group P21.35 For the larger Sn4+ ion, all of the rareearth stannates form in the cubic phase even with the largest La3+ ion. A recent study of stannate single crystals was reported,36 but most stannates have been studied as polycrystalline powders.12 We recently found that rare-earth stannates could be prepared as high-quality single crystals using a high-temperature (700 °C) hydrothermal method, and the physical and magnetic properties of these crystals are currently under investigation.37 The Ge4+ ion is considerably smaller than the Ti4+ and Sn4+ ions, so the A/B ratio is greater than 1.84 in all cases, and the cubic pyrochlore phase is generally not stable for any rare-earth germanate. The smaller RE analogues (Eu−Lu) form a complex tetragonal structure that appears to be the most thermodynamically stable phase, while the larger RE ions adopt lowersymmetry P1̅ structures.38 To access the cubic pyrochlore rareearth germanates, the group led by Wiebe performed a series of high-pressure syntheses on the smaller rare-earth germanates.39,40 They were able to prepare stable, phase-pure powders of RE2Ge2O7 (RE = Dy, Er, Ho, Yb) at a pressure of 7 GPa at 1000 °C.33 While Yb2Ge2O7 appears to exhibit antiferromagnetic order, exotic spin-dynamic behavior has been shown for Yb2Ti2O7 and Yb2Sn2O7, which strongly suggested the need for a further detailed examination of the pyrochlores as a general class of materials.39−43 We developed a high-temperature hydrothermal reaction method (600−700 °C) to grow high-quality single crystals of a number of extremely refractory oxides,44 including Lu2O3,45,46 HfO2,47 ThO2,47,48 and rare-earth stannate pyrochlores.37 Refractory oxides are far more commonly prepared as ceramic powders than single crystals. Single-crystal growth by classical methods (Czochralski pulling, flux growth, top-seed solution growth, and optical floating-zone growth) requires very hightemperature conditions that can induce many lattice imperfections such as oxide defects, impurities, and inhomogeneities. Another problem, particularly acute in pyrochlores, is a tendency for A/B site disorder. These defects and site disorders can lead to ambiguities in the physical data from sample to sample.49,50 This issue creates a Catch-22 scenario, where the choice is often between relatively pure powders or defect-containing crystals, making a low-temperature single-crystal growth method highly desirable. The relatively low-temperature hydrothermal growth

method described herein leads to relatively large single crystals (1−2 mm), while minimizing both defects and site disorder. Earlier Demyanets et al. reported the hydrothermal synthesis of cubic pyrochlore Yb2Ge2O7 single crystals for the first time. The product was prepared in an extremely small stability field in an otherwise complicated phase diagram, but few subsequent details were provided.51,52 We found that this chemistry is very complex, and many rare-earth germanates can be synthesized from hydrothermal fluids. In this preliminary Communication, we report that large, high-quality single crystals of RE2Ge2O7 (RE = Yb and Lu) can be grown under the appropriate hydrothermal conditions (Figure 2; experimental details in the

Figure 2. Optical micrographs of hydrothermally grown cubic (a) Yb2Ge2O7 and (b) Lu2Ge2O7.

Supporting Information). In high (10 M) concentations of mineralizer, the tetragonal structure of Yb2Ge2O7 forms in high yield (eq 1). When the concentration of the KF mineralizer is reduced to 1 M (eq 2), however, relatively large crystals (1−2 mm) of Yb2Ge2O7 are formed in the desired cubic pyrochlore structure type. We are not clear why this difference in the structure type occurs, but the process is reproducible in high yield for both polytypes. We find that the corresponding cubic Lu2Ge2O7 can also be prepared under the same reaction conditions (650°C/200 MPa). It is observed that the synthesis is reproducible for other alkali fluorides but thus far only seems to work for the smallest two RE ions. For the RE elements La−Nd, the triclinic phase RE4(Ge3O10)(GeO4) forms, while reactions with Sm−Tm form the tetragonal RE2Ge2O7 phase (regardless of the mineralizer concentration). Yb2 O3 + 2GeO2 → Yb2 Ge2O7 (P 41212)

10 M KF

(1)

Yb2 O3 + 2GeO2 → Yb2 Ge2O7 (Fd 3̅ m)

1 M KF

(2)

These crystals are refined to a very high precision by singlecrystal X-ray diffraction (XRD; Tables S1 and S2). The room temperature unit cell parameters are a = 9.8297(7) Å and V = 949.78(6) Å for Yb2Ge2O7. The Yb3+ ions occupy the eightcoordinate 16d Wyckoff compressed rhomb site [Yb−O = 2.12819(15) Å (axial) and 2.4303(17) Å (equatorial)]. The large anisotropy of the RE−O bond lengths indicates compression of the REO8 cube under chemical pressure from the small B-site Ge4+ ions. Such anisotropy appears to be a significant contributor to the magnetic properties in cubic pyrochlores.53 The growth of relatively large single crystals here becomes especially significant, enabling magnetic measurements on well-oriented single crystals to probe the magnetism in additional detail. Careful inspection of the anisotropic displacement parameters, coupled with test refinements of the site occupancies B

DOI: 10.1021/acs.inorgchem.8b01810 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

transition at 572 mK (reported as 570 mK in ref 39 and 620 mK in ref 42). This confirms the high quality of our single crystals because even small amounts of lattice disorder dramatically affect the sharpness and temperature of the first-order transition in the related compound Yb2Ti2O7.49 The specific heat of Yb2Ge2O7 also shows a broad hump near 3 K, which is observed in the other Yb pyrochlores, signaling a crossover to a magnetically strongly correlated regime. The first-order transition signals the development of antiferromagnetic longrange order in Yb2Ge2O7,42 which is of great interest because of its unusual excitation spectrum.9 In summary, we discovered suitable conditions for the growth of single crystals of the cubic pyrochlore Yb2Ge2O7, and the crystal quality was confirmed using both single-crystal XRD and heat capacity measurements. This breakthrough enables a more detailed examination of the magnetic structure of this material using single-crystal neutron diffraction both in a magnetic field and in zero field, at low temperatures. Previous work demonstrated interesting results on the cubic Yb2 B 2 O 7 pyrochlores (B = Ge, Sn), resulting from spin frustration at low temperature and hinting at the formation of new forms of matter. Access to high-quality single crystals enables singlecrystal neutron studies to further explore these results.

refined as free variables, suggests that there is no site disorder in the crystals. This is further confirmed by the heat capacity data, as discussed below. There is also no significant residual electron density to suggest a stuffed lattice. As such, these well-ordered cubic pyrochlore single crystals are extremely useful for the detailed magnetic and neutron investigations now underway. Low-temperature single-crystal XRD data collections on Yb2Ge2O7 produced a significant decrease in the lattice parameter with a = 9.8186(11) Å at 150 K and a nonlinear temperature variation of the lattice size from 150 to 300 K (Figure S1). Variation of the lattice parameter as a function of the temperature is mirrored most closely by variation of the axial Yb−O distance, which is the Yb−O bond aligned along the body diagonal. This is a result of rhombic compression of the REO8 cubes and may be why the small lattice parameter of the germanate pyrochlores is such a strong indicator of their unusual magnetic behavior. The formation of the cubic phase from hydrothermal fluids may be due to the pressure in the autoclaves, but at this point, we reject that postulate for several reasons. We feel that the pressure in the autoclaves is simply too small compared to the induced pressures employed by Hallas et al., which are typically 7 GPa. The pressures in our system (ca. 200 MPa) are 30 times less than those values.40 We never observed any pressure-induced phase transitions in any of the many hydrothermal reactions that we performed over the last 20 years. Further, the pressure in the system is autogenous, meaning that it is generated internally simply by the vapor pressure of the heated fluid, so no external pressure is actually applied. Also, the reactions using 1 and 10 M KF were done at essentially the same pressure, but the two different phases were observed in the quantitative yield in each case. This strongly implies that the choice of phase is a function of the chemical environment. Furthermore, Hallas et al. were able to use pressure to obtain many cubic rare-earth germanates, in contrast to our inability to observe cubic phases for any ion larger than Yb3+ in the germanate system. This suggests that we are merely on the very cusp of stability for the cubic phase and that the relatively low reaction temperature enables capture of the cubic phase. We also found that the cubic pyrochlore phase, once formed, was retained upon heating to 1000 °C in air, with no phase change or degradation of the crystal quality. We obtained the specific heat capacity (Figure 3) of the crystals, which agrees well with the published data on powder samples of the same material, including a sharp first-order

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b01810. Experimental details, tables of crystallographic data and bond lengths, and temperature dependence of structure parameters (PDF) Accession Codes

CCDC 1851404−1851405 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Colin D. McMillen: 0000-0002-7773-8797 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Department of Energy Basic Energy Sciences (Grant DE-SC0014271) for financial support. K.A.R. acknowledges funding from the CIFAR.

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REFERENCES

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Figure 3. Specific heat of hydrothermally grown Yb2Ge2O7 single crystals displaying a sharp first-order transition at 572 mK and a broad hump near 3 K. The upturn near 10 K is due to the lattice contribution to the specific heat, which has not been subtracted. C

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DOI: 10.1021/acs.inorgchem.8b01810 Inorg. Chem. XXXX, XXX, XXX−XXX