Hydrothermal Crystal Growth of Rare Earth Tin Cubic Pyrochlores

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Hydrothermal Crystal Growth of Rare Earth Tin Cubic Pyrochlores, RE2Sn2O7 (RE = La-Lu): Site Ordered, Low Defect Single Crystals Matthew Powell, Liurukara D. Sanjeewa, Colin D. McMillen, Kate A. Ross, Colin L. Sarkis, and Joseph W. Kolis Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b01889 • Publication Date (Web): 26 Jul 2019 Downloaded from pubs.acs.org on July 30, 2019

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Crystal Growth & Design

Hydrothermal Crystal Growth of Rare Earth Tin Cubic Pyrochlores, RE2Sn2O7 (RE = La-Lu): Site Ordered, Low Defect Single Crystals

Matthew Powell1, Liurukara D. Sanjeewa1, Colin D. McMillen1, Kate A. Ross2, Colin L. Sarkis2, Joseph W Kolis*1

1. Department of Chemistry, Clemson University, Clemson, SC 2. Department of Physics, Colorado State University, Fort Collins, CO.

Abstract: A hydrothermal route to single crystals of rare earth stannates RE2Sn2O7 (RE = La-Lu) in the cubic pyrochlore structure is reported. Growth reactions were performed in aqueous fluids at 700˚C and 200 MPa with CsF mineralizers in concentrations ranging from 0-30 M, with 20 M CsF providing the most consistent results. Single crystals of the entire range of lanthanides were grown and characterized by single crystal x-ray diffraction and found to be isostructural in the Fd-3m space group. The unit cell sizes range from 10.7106(16)Å for La2Sn2O7 to 10.3005(9)Å for Lu2Sn2O7. Both the unit cell size and RE-O distances are found to be essentially linear with respect to the ionic radius of the rare earth ion. The high quality diffraction data strongly suggests that there is very little site disorder or lattice defects in the sample. Of particular interest is the synthesis and single crystal growth of Ce2Sn2O7, which represents one of the few f1 pyrochlore samples. Specific heat measurements were obtained down to 50 mK on both

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Yb2Sn2O7 and Ce2Sn2O7, with the Yb material displaying a single sharp peak at 138 mK suggesting that the sample does not contain any site disorder or “ lattice stuffing”. The Ce analog shows only a broad featureless transition below 100 mK, with this lack of long-range magnetic order consistent with quantum spin liquid behavior.

1. Introduction: Compounds with the generic formula A2B2O7 adopt a number of different structural types ranging from triclinic to cubic, but the structures with the richest and most interesting physical and magnetic properties are the pyrochlores with a cubic structure (Fd-3m).1,2 In this case both A and B metal ions have (-)3m site symmetry, although each site has a very different coordination environment. The metal ion on the B site has a fairly conventional six coordinate distorted octahedral environment, but the A site has an unusual eight coordinate environment resulting from the compression of the body diagonal rhombohedral axis of a cube, leading to two short trans RE-O bonds and six longer RE-O bonds around the waist of the rhomb.1 A unique aspect of the structure is the formation of tetrahedral superstructure of metal ions of both the A and B ions.2 There are no direct bonds between the metal ions, rather the tetrahedral clusters are magnetically coupled by oxo bridges between the metal centers. These tetrahedra possess threefold symmetry around the body-diagonals of the unit cell, which generates an excellent example of a 3-D spin frustrated system.3 This complex 3-D frustration gives rise to an enormous range of unusual magnetic states, including the suggestion of quantum spin states that would represent new forms of matter.3.4


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Our interest is focused on the general class of cubic pyrochlores with a magnetically active rare earth trivalent ion in the A position and a magnetically silent tetravalent ion (e.g. Ti4+, Zr4+, Sn4+, Ge4+) on the B site. In some cases the tetrahedra of magnetic rare earth ions on the A site can form an unusual two spin-in and two spin-out arrangement of the magnetic vectors at each corner of the tetrahedra.5.6 Given the three fold structural symmetry of the tetrahedra and the two in-two out arrangement of the spin vectors, a range of spin frustration behavior can be observed. In particular the placement of two types of vectors on the three-fold site is reminiscent of the arrangement of two O-H covalent bonds and two O…H hydrogen bonds on a three-fold site in the lattice of crystalline ice, as first observed by Pauling.7 Thus the corresponding magnetic vector arrangement is called a “spin-ice”.6 The overall effect of such a structural conundrum is to create a massive number of non-ordered ground states leading to a residual entropy value even at the theoretical minimum of absolute zero (zero point entropy).8 These anomalies lead to quantum spin liquids and other exotic phases that suggest new forms of matter.9,10 Because of these factors the cubic pyrochlores where A = rare earth, and B = an electronically silent tetravalent ion, have been the focus of intense inquiry in the last several years. ,9,11,12 In general the stability field of the cubic pyrochlore phase is dictated by the ionic radius ratio of A and B, where the ratio must be smaller than a critical value to stabilize the cubic phase versus lower symmetry structure types. Originally it was thought that the critical value was a straightforward ratio of A vs. B,1 but later work suggests that a more complex relationship may be needed to provide a more reliable indicator.13,14 The most common and heavily studied members of this category are the heavier rare earth titanates RE2Ti2O7 (RE = Sm-Lu) in part because they are extremely stable and readily form single crystals via a variety of melt growth techniques.15 It should be noted that because of the relatively small size of the Ti4+ ion


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(0.605A),16 the cubic pyrochlore structure is only stable for the smaller rare earth ions (Sm-Lu), while the larger rare earth titanates (La-Nd) are only stable as polar monoclinic phases.17,18 In contrast to the titanates, the corresponding germanates RE2Ge2O7 have a much more complex phase behavior.19 Because of the smaller size of the germanate ion (0.530Å) they are extremely sensitive to the ionic radius of the rare earth and form a wide range of structure types. In general the smaller, heavier rare earths (Gd-Lu) do not lead to the cubic phase. Rather they tend to form a tetragonal phase in the unusual P41212 space group, while a triclinic phase with a complex structure forms with the lighter, larger rare earth ions (La-Nd).19 The importance of the frustrated magnetic phases suggesting quantum spin behavior generates considerable interest in synthesis of the cubic phase of the rare earth germanates. This was recently achieved in the form of powder material using extremely high-pressure methods (7GP at 1000˚C). This synthesis enabled the initial magnetic and neutron diffraction experiments with the rare earth germanate cubic pyrochlores.20-23 In these cases a range of magnetic behaviors can be observed which appear to be a sensitive function of the B ionic radius and resultant unit cell size, as well as the magnetic spin of the rare earth ions. Interestingly we recently found that we could employ hydrothermal synthesis as a route to relatively large single crystals of RE2Ge2O7 (Yb, Lu) that grow in the cubic phase.24 At the opposite end of the size spectrum, the larger B4+ ions such as zirconate (Zr4+ ionic radius = 0.72Å) appear to only form cubic pyrochlores for the largest rare earth ions (La-Sm),25,26 while the smaller rare earths (Eu-Lu) tend to form defect fluorites and the poorly understood -phase A4B3O12.27 Among the more interesting rare earth cubic pyrochlores are those containing the intermediate sized Sn4+ (0.690Å) as the B ion, which display some unique properties. For example, Ho2Sn2O7 and Dy2Sn2O7 are among the first observed examples of pyrochlore spin


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ices.28,29 The stannates are especially unique in that, for the various spin silent B-site tetravalent building blocks, Sn4+ is the only one thus far observed to form the cubic pyrochlore phase for all of the rare earth ions. It has the ideal ionic radius to stabilize the cubic phase for the largest to smallest rare earth ions. Thus the stannates represent a unique opportunity to make direct comparisons of physical properties of cubic pyrochlores across the entire rare earth spectrum. A number of methods are reported for the synthesis of the rare earth stannates,30 but there are some significant challenges in their synthesis. At high temperatures Sn4+ ions tend to become reduced by extruding oxygen, creating oxide defects in the lattice. In particular traditional single crystal growth of rare earth stannates is problematic as the stannates are generally quite refractory, requiring high temperatures for crystal growth. This leads to reduced stannate ions as well as oxide defects in the lattice. In fact the growth of “simple” SnO2 itself as a single crystal is not straightforward and requires considerable effort to minimize the defect density.31 A further problem for the pyrochlores arises because the high temperatures required for classical melt based crystal growth also induce site disorder, with the rare earth and tin ions disordering over the A and B sites. Because of these issues it is difficult to get single crystals of suitable quality for detailed magnetic and neutron diffraction studies. In particular there have been relatively few single crystal studies of the rare earth stannates.32,33 Thus many of the magnetic and physical property studies of the pyrochlore stannates have been performed on powders.28 Given the interest in the physical properties of this system it is desirable to have a route to single crystals at sufficiently low temperature to minimize tin reduction, oxide defects and site disorder. In instances such as these, it is particularly valuable to establish detailed structural data for pyrochlore material prepared by significantly different synthetic techniques than those already in the literature.1,2,13,14,30,32-36


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Recently we developed a high temperature hydrothermal method as a route to single crystals of refractory oxides.37 We found that, under suitable conditions using the appropriate mineralizers, we can grow reasonably sized single crystals of otherwise very recalcitrant metal oxides. These include oxides of tantalum, thorium and most of the rare earths.38-40 Typically we can obtain solubility, transport and single crystal growth in aqueous fluids at temperatures between 600 and 700˚C and pressures of 150-200 MPa using concentrated hydroxides or fluorides as the mineralizer. One particular advantage to this approach is that the growth processes occur at relatively low temperatures compared to other oxide growth methods such as floating zone growth in an optical furnace,41 which can often result in oxide defects and site disorder.26,42,43 It should be noted that even for the case of rare earth titanates where the stability of the phase is well established, lattice defects and nonstoichiometry can still present problems for crystal grown by melt techniques.44 Hydrothermal growth at much lower temperatures in sealed systems tends to minimize both oxide lattice defects and site disorder. We are especially attracted to the pyrochlore series because of our success in solubilizing and transporting rare earth oxides. Given our earlier success in the growth of high quality single crystals of the rare earth sesquioxides we felt the pyrochlores represent a natural extension of this crystal growth approach. In this paper we describe the synthesis of the entire series of rare earth tin pyrochlores as well formed single crystals with no evidence of any site disorder. This work paves the way for the detailed study of the magnetic and physical properties by various methods especially single crystal neutron diffraction. In particular it describes the high yield synthesis of the cerium analog, Ce2Sn2O7 that provides access to an f1 system with potential quantum spin liquid behavior.45-47


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2. Experimental Methods 2.1. Synthesis and Crystal Growth of Ce2Sn2O7: Single crystals of Ce2Sn2O7 were grown starting from CeO2 and SnO2 powders in varying concentrations of CsF ranging from 0-30 M. Stoichiometric amounts of CeO2 (Strem, 99.99%) and SnO2 (Strem, 99.99%) were weighed and added to 1/4 in. outer diameter silver ampoules 2.75 in. in length. CeF3 was also tested to assess how starting with trivalent Ce3+ impacts growth as well as to assess its capability as a potential starting material. The choice to utilize tetravalent Ce4+ in the form of CeO2 is explained in detail later. The CsF mineralizers were prepared by weighing a calculated amount CsF powder (Alfa Aesar, 99.9%) into a silver ampoule, and adding the appropriate amount of deionized water to obtain the required concentration. Typically, a total of 0.15 to 0.2 g of solid powder was mixed with 0.4 mL of mineralizer. An Ar TIG welder was used to weld seal the ampoules before being placed into a 27 mL internal volume Inconel Tuttle cold-seal style autoclave. An appropriate amount of deionized water was added to provide suitable counterpressure to prevent the ampoules from bursting. The autoclaves were heated to 700 °C and pressures between 110-140 MPa (approximately 16,000-20,000 PSI). Durations ranged from 24 hrs, which yield small single crystals averaging 0.1-0.3 mm, to 14 days to yield larger well-defined crystals approximately 0.4-0.5 mm in size. Following heating, the autoclave was cooled to room temperature over several hours before the ampoules were collected and vacuum filtered.

2.2. General Synthesis of RE2Sn2O7: General preparation of other RE2Sn2O7 pyrochlores followed a similar synthesis procedure with the mineralizer concentrations primarily being 20-30 M CsF to achieve similar results. With the exception of Tb2Sn2O7, which used the mixed Tb(III)/Tb(IV) oxide Tb4O7 as


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the rare earth feedstock, the other RE stannates were synthesized from RE2O3 powders (99.9% Alfa Aesar, HEFA). Total amounts of feedstock powders and relative ratios were employed as with the cerium materials, along with the reaction time and temperature described above.

2.3. Growth of Larger Crystals of RE2Sn2O7: Larger single crystals of rare earth stannate pyrochlores (1-2 mm) were grown using seed crystals obtained from the above spontaneous nucleation experiments. As a general protocol, Ce2Sn2O7 crystals >1 mm were grown using 6 in. long ampoules made of silver tubing. In the specific case of Ce2Sn2O7, tubing diameter was 1/4 in. in order to minimize surface area available for potential silver oxidation resulting from the reduction of Ce(IV) to Ce(III). A small crimp was introduced at the midpoint of the ampoule to separate the hot zone containing the feedstock oxides and the cold growth zone that housed the seed crystals. Various concentrations of CsF were employed as the mineralizer, but 20 M typically provided the most consistent results. Growth was typically performed with the hot zone at 720 °C and the cold zone at 700 °C to provide a 20 °C gradient to facilitate mass transport. Growth runs typically were 2-3 weeks Other RE2Sn2O7 crystals (i. e RE = Pr, Yb) were grown in 3/8 in. diameter silver tubing of the same length (6.5-7 in.) since there is no redox chemistry involved in growth. The general setup followed that of Ce with seed crystals from previous smaller reactions being deposited in the cold zone to provide a pre-made substrate upon which further growth could occur as mass is transported upwards during the reaction.

2.4. Characterization:


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Single crystal X-ray diffraction was performed on each member of the RE2Sn2O7 family at room temperature using a Bruker D8 Venture diffractometer with Mo Kα radiation (λ = 0.71073 Å) and a Photon 100 detector. Instrument control, data processing (SAINT), and data scaling (SADABS) was performed through the Apex 3 software system.48 The structures were solved by intrinsic phasing (SHELXT) and further refined by full matrix least squares techniques on F2 (SHELXL) using the SHELXTL suite.49 Substitutional disorder of the A and B ion sites was tested during the course of the structure refinement by allowing mixed occupancy of the sites and allowing the occupancy values to refine as free variables. In all cases, the A site was found to be occupied solely by the rare earth ion, while the B site was solely occupied by Sn4+. All atoms were refined anisotropically. Details of the structure refinements are provided in the Supporting Information, Table S1-S3. The heat capacity at constant pressure (Cp(T)) of single crystals of Yb2Sn2O7 and Ce2Sn2O7 were measured from 1 K down to 50 mK by a thermal relaxation method, using a dilution insert in a Quantum Design Physical Properties Measurement System. To achieve higher point density over the sharp, low temperature anomaly in Yb2Sn2O7, a “long pulse” method was used and analyzed as described elsewhere.50

3. Results and Discussion 3.1. Crystal Growth: Here we describe the hydrothermal synthesis and crystal growth of the entire series of the entire rare earth series (La-Lu) of stannates RE2Sn2O7 in the cubic pyrochlore structure type. To the best of our knowledge tin is the only example of a closed shell tetravalent B site ion to stabilize the cubic pyrochlore structure across the entire rare earth series, as Ge and Ti are too


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small to stabilize the largest rare earth ions (La-Nd), while Zr, Hf etc. are too large to stabilize the smallest rare earth ions (Gd-Lu). In all cases reported here, the stannates form as high quality single crystals. We demonstrate that relatively large (1-2mm) single crystals can be grown for most of the rare earths. It is important to note that the materials are grown at relatively modest temperatures (650-700˚C) in a closed system. This leads to minimal oxide defects in the lattice because the tin ions remain exclusively in the tetravalent state. Also we see no evidence of site disorder among the rare earths and the tin site in the lattice (vida infra). This lack of disorder and defects in the single crystals greatly enhances the ability to obtain high quality physical property measurements. Most of the rare earth stannates can be synthesized as pure crystals in nearly quantitative yields by the simple reaction of the rare earth sequioxides RE2O3 with SnO2 at 700˚C and 150-200 MPa using 5-10 M CsF as a mineralizer for seven days. Representative examples are shown in Figure 1. An excess of SnO2 is useful to minimize the tendency to form any deleterious REOOH which can sometimes occur during cool down of the reaction. These conditions generally produce well-formed single crystals of approximately 1 mm in size. Longer reaction times and careful control of thermal gradients within the autoclaves can lead to larger crystals if desired.


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Figure 1. View of several of the rare earth stannate pyrochlore crystals prepared hydrothermally. Left to right: Ce2Sn2O7, Er2Sn2O7, Pr2Sn2O7 and Tm2Sn2O7 respectively. The reader is directed to reference 37 for a detailed review of the crystal growth apparatus. One of the more unique members of this series is the Ce3+ analog Ce2Sn2O7. It has a number of interesting aspects to consider. This particular material was previously reported as an impure powder but we prepared it as high quality, extremely pure single crystals using a hydrothermal reaction. In the unique case of cerium we assume that the source of reducing electrons is the wall of the silver tubes (Eq. 1). This reaction leads to very good yields of high quality single crystals (Figure 2).

CeO2 + SnO2 + 1e-  ½ Ce2Sn2O7 700˚C 150MPa 10-30 M CsF

(1)

Figure 2. The composite reaction product of hydrothermal Ce2Sn2O7 synthesis.

Ce3+ is of particular interest as it is an f1 system, so it is the Hund’s 3rd rule opposite of Yb3+ f13 complexes. Preliminary earlier work suggests that this may be a good example of a quantum spin liquid.45 The ability to prepare pure single crystals enables the possibility of careful examination of physical properties using magnetic studies, neutron diffraction and other methods. The ability to employ relatively low temperature synthesis in closed systems helps stabilize the Ce3+ ion since it otherwise has a marked tendency to revert to Ce4+ in many systems.


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The use of a unique catalytic redox reaction in the synthesis herein was found to provide the most satisfactory results in this particular case, but the straightforward use of Ce3+ (i.e CeF3 or Ce2O3) as a starting material should provide access to a wide range of Ce3+ f1 solids for subsequent study.

3.2. Structural Data: We performed single crystal X-ray diffraction studies on all the rare earth stannates and found that in all cases the scattering factors suggest highly ordered structures. The R factors are very low in all cases and the anisotropic displacement parameters are within a consistent range. Given the quality of the data collected and the difference in scattering factors for the heavy atoms it is highly likely that any site disorder would be detected by X-ray scattering and none is observed for any of the samples suggesting little if any site disorder. Similarly the coloration of the crystals is always characteristic of the lanthanide ion present, and the absence of alternative, dark coloration also strongly suggests minimal oxide lattice defects. Typically the high temperature synthesis required for single crystals of rare earth stannates lead to darkly colored material with increased conductivity due to oxide defects.30 Specific heat measurements as detailed below also suggest that there is minimal site disorder or lattice defects in these materials. Evaluation of the lattice parameters is also a useful way to evaluate the extent of site disorder or stuffing in pyrochlores. In particular, a measurable increase in the lattice parameter has been shown to accompany lanthanide stuffing at the tetravalent B-site in titanate pyrochlores.42,51 An increase in lattice parameter of 0.02 to 0.03 Å over the lattice parameter for a stoichiometric sample was observed for Ln2(Ti2-xLnx)O7-x/2, where x was only 0.1 (the lattice parameter increased on the order of 0.2 to 0.3 Å when x = 0.67).51 Although the stannate


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pyrochlores are less studied than the titanates, there are still a few opportunities to compare the lattice parameters obtained from hydrothermally-grown single crystals in the present study (Table 1) to those prepared by other techniques.

For example, a = 10.6420(7) Å for

hydrothermal Ce2Sn2O7 determined by single crystal XRD, versus a = 10.6547(2) Å from room temperature powder XRD measurements of Ce2Sn2O7 prepared by solid state techniques.46 Additional comparison of the lattice parameters of the hydrothermal crystals in Table 1 may be made to those of the flux-grown single crystals of Pr2Sn2O7 (10.63 Å), Tb2Sn2O7 (10.46 Å), Dy2Sn2O7 (10.44 Å), and Ho2Sn2O7 (10.37 Å), which were proposed to have oxygen deficiency or stuffing defects.30 The smaller lattice parameters of the hydrothermal crystals for a given pyrochlore is further suggestive of their low defect content, in accord with the specific heat measurements below. The high quality single crystal structures of all these materials also provide useful archival insight into the lattice parameters and interatomic distances in the lattices (Table 1). We observe that there is a linear relationship between ionic radius of the rare earth ion and the unit cell parameter ranging from 10.7106(16) Å for La2Sn2O7 to 10.3005(9) Å for Lu2Sn2O7. Similarly there is a linear relationship in the RE-O distances with the six hexagonal RE-O distances ranging from 2.631(2) Å to 2.466(2) Å of the rare earth oxide rhomb, while the shorter trans distances range from 2.3189(4) Å to 2.2301(2) Å. (Figure 3) Such regular relationships should be useful in any systematic investigation of an isostructural system encompassing all the rare earths.


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Table 1: Structural parameters and R factor (observed data) of hydrothermally-grown rare earth stannate pyrochlores determined by single crystal X-ray diffraction. Formula

a (Å)

RE−O1 (Å)

RE−O2 (Å)

Sn−O1 (Å)

R1

La2Sn2O7 Ce2Sn2O7 Pr2Sn2O7 Nd2Sn2O7 Sm2Sn2O7 Eu2Sn2O7 Gd2Sn2O7 Tb2Sn2O7 Dy2Sn2O7 Ho2Sn2O7 Er2Sn2O7 Tm2Sn2O7 Yb2Sn2O7 Lu2Sn2O7

10.7106(16) 10.6420(7) 10.6196(12) 10.5749(11) 10.520(2) 10.4897(11) 10.4578(13) 10.4287(9) 10.4063(6) 10.3777(15) 10.3616(6) 10.3278(8) 10.3146(12) 10.3005(9)

2.6304(15) 2.6040(16) 2.593(2) 2.579(2) 2.554(3) 2.544(2) 2.533(3) 2.518(3) 2.510(2) 2.495(4) 2.488(3) 2.482(3) 2.474(3) 2.466(2)

2.3189(4) 2.3041(2) 2.2992(3) 2.2895(2) 2.2777(4) 2.2711(2) 2.2642(3) 2.2579(2) 2.2530(1) 2.2468(3) 2.2434(1) 2.2360(2) 2.2332(3) 2.2301(2)

2.0761(9) 2.0685(10) 2.0677(13) 2.0607(14) 2.057(2) 2.0528(12) 2.0489(15) 2.048(2) 2.0456(13) 2.045(2) 2.0437(18) 2.036(2) 2.036(2) 2.0365(15)

0.0133 0.0111 0.0129 0.0168 0.0187 0.0124 0.0189 0.0215 0.0115 0.0196 0.0182 0.0195 0.0262 0.0118


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Figure 3. Dependence of lattice parameter (top), long RE-O (middle) and short RE-O (bottom) distances on rare earth ionic radii in the RE2Sn2O7 system.

3.3. Specific Heat Measurements: The compound Yb2Sn2O7 displays a sharp anomaly in at 138 mK (Figure 4). This agrees with the specific heat previously reported in polycrystalline samples.52 This agreement alleviates any concerns about small levels of “stuffing” (site disorder as a result of Yb occupying the Sn site) that was observed in some single crystals of Yb2Ti2O7, which had a profound effect on the magnetic properties of that compound.42 This transition has previously been identified as leading


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to ferromagnetic order with unusual spin excitations.23 In stark contrast to Yb2Sn2O7, Ce2Sn2O7 shows a broad feature that indicates a lack of a transition to long range magnetic order. This is consistent with its identification as a spin liquid candidate.45 The change in entropy, ∆S = 1𝐾

𝐶𝑝

∫0.06𝐾 𝑇 𝑑𝑇, associated with the specific heat below 1K is shown as an inset to Fig. 4. Both Yb2Sn2O7 and Ce2Sn2O7 display entropy changes that approach the theoretically expected Rln2 (consistent with effective spin ½ moments). The “missing entropy” is the result of not having measured to lower temperatures for Ce2Sn2O7 (at our base temperature of 60 mK there is still significant specific heat), or higher temperatures for Yb2Sn2O7 (a broad hump in specific heat is known to be present around 4K in this material23, corresponding to short range magnetic order). Powder X-ray diffraction patterns of hydrothermally-grown Ce2Sn2O7 and Yb2Sn2O7 are given in the Supporting Information, Figure S1.

Figure 4. Specific heat at constant pressure for Ce2Sn2O7 and Yb2Sn2O7. Inset: change in entropy from the lowest temperature measured to the 1K. Both Yb2Sn2O7 and Ce2Sn2O7 approach Rln2 as expected for their effective spin ½ moments.


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4. Summary and Conclusions: In this work we demonstrate the synthesis and single crystal growth of the entire series of rare earth stannate cubic pyrochlores. We employ a hydrothermal growth method using 5-20 M CsF mineralizer at 700˚C and 150-200 MPa. This represents, to our knowledge, the only well characterized complete series of the RE2M2O7 (M = empty shell tetravalent ion, Ge4+, Ti4+, Sn4+, Zr4+) single crystals, where the complete series of rare earth analogs can be isolated as the cubic Fd-3m structure. The modest growth temperature provides a number of advantages over classical (higher temperature) growth methods. The traditional challenges associated with growth of rare earth stannate pyrochlores are two-fold. One is the tendency of any stannates to extrude oxygen creating reduced tin centers and defects in the oxide lattice. The other is the penchant for the rare earth and Sn4+ ions to site disorder over the two RE3+ and M4+ sites of the pyrochlore lattice. Both problems are particularly relevant for the rare earth stannate pyrochlore and create significant complications for the detailed interpretation of complex physical properties of the magnetically frustrated pyrochlores.42 In general both problems arise from the high temperatures necessary to synthesize single crystals of the extremely refractory rare earth stannates. With the use of hydrothermal growth methods, the reactions are performed at relatively low temperatures in a sealed system. These two factors combine to minimize the problems described above and lead to well-ordered materials with few oxide lattice defects. In addition the ability to transport solubilized feedstock across a thermal gradient leads to growth of single crystals of reasonable size that enables detailed physical property measurements including single crystal neutron diffraction and physical property measurements of single crystals oriented in a magnetic field. This technique led to relatively large (0.5-2 mm) single crystals of essentially all of the rare earth stannate cubic pyrochlores. Larger crystals can be prepared by reaction over a


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longer period of time or by variation of thermal gradient and mineralizer, as desired. High quality single crystal diffraction data was obtained for all the rare earth stannates. Careful examination of the single crystal scattering data showed no unusual or unexpected behavior of the anisotropic displacement parameters or scattering factors, strongly suggesting that there is no site disorder or significant lattice defects in the crystals. Also the crystals are generally colorless or lightly colored in their bulk, again strongly suggesting little if any oxide lattice defects. A plot of unit cell size versus rare earth ionic radii shows linear behavior. Similar linear behavior is observed for plots of the two unique RE-O distances. Perhaps the most unique member of the rare earth stannates in this work is the cerium analog Ce2Sn2O7 that can be prepared cleanly as large bright yellow single crystals. It has previously been reported as a somewhat contaminated powder but showed considerable promise as a quantum spin liquid as an f1 frustrated magnet. Thus the ability to prepared high quality single crystals opens the door for more detailed single crystal magnetic and neutron diffraction studies that are currently underway. In addition, the control of stoichiometry redox potential and reaction conditions should enable a more general investigation of other Ce3+ oxides that are somewhat rare but may also display unusual physical properties.

The specific heat

measurements of Yb2Sn2O7 and Ce2Sn2O7 show evidence of a ferromagnetic transition in the former (previously observed in polycrystalline samples, and known to have unusual magnetic excitations), and candidate quantum spin liquid behavior in the latter. The availability of single crystals of these compounds will enable new measurements to shed light on their unusual quantum phenomena. The ability to perform precision synthesis and controlled crystal growth of such otherwise refractory oxides also opens the door for a more detailed examination of phase space of these


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exciting frustrated oxides. For example we now have the ability to grow single crystals of any of the rare earth pyrochlores containing a linear combination of dopants (Ge, Ti, Sn, Pb, Pt etc) at the B site. This will allow access to richer and more complex phase diagrams of spin frustrated pyrochlore and hopefully ultimately the rational designed synthesis of new quantum materials. ASSOCIATED CONTENT: Supporting Information Crystallographic refinement data from single crystal X-ray diffraction, powder X-ray diffraction patterns of Ce2Sn2O7 and Yb2Sn2O7. Accession Codes CCDC 1885528-1885541 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.

ACKNOWLEDGEMENT: The synthesis, crystal growth and crystallographic studies were performed at Clemson University and supported by DoE BES Award Number DE-SC0014271.

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For Table of Contents Use Only:

Hydrothermal Crystal Growth of Rare Earth Tin Cubic Pyrochlores, RE2Sn2O7 (RE = La-Lu): Site Ordered, Low Defect Single Crystals Matthew Powell, Liurukara D. Sanjeewa, Colin D. McMillen, Kate A. Ross, Colin L. Sarkis, Joseph W Kolis*

Single crystals of the rare earth stannate pyrochlores have been grown hydrothermally. Single crystal X-ray diffraction measurements reveal the crystals to be high quality with no discernable site disorder, further confirmed by heat capacity measurements. The Ce2Sn2O7 system is a potential quantum spin liquid material.


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Hydrothermal Crystal Growth of Rare Earth Tin Cubic Pyrochlores

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