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Growth, Structural Considerations, and Characterization of CeDoped (La,Gd)2Si2O7 Scintillating Crystals Akira Yoshikawa,*,†,‡,§ Shunsuke Kurosawa,†,‡ Yasuhiro Shoji,†,§ Valery I. Chani,† Kei Kamada,‡,§ Yuui Yokota,‡ and Yuji Ohashi† †

Institute for Materials Research (IMR), Tohoku University, Sendai, 980-8577, Japan New Industry Creation Hatchery Center, Tohoku University, Sendai, 980-8579, Japan § C&A Corporation, Sendai, 980-8579, Japan ‡

ABSTRACT: Ce-doped lantanium-gadolinium pyrosilicate (La,Ce,Gd)2Si2O7 (Ce:La-GPS), crystals with various content of rare-earth elements were produced from the melt, and their optimal La/Gd ratio was examined. It was found that Ce:La-GPS single crystals of acceptable optical quality can be produced from the melts ranging from La0.5Gd1.5Si2O7 to La1.0Gd1.0Si2O7 and containing about 1 atom % Ce with respect to the host rare-earths of La and Gd. The crystal growth was performed by the micro-pulling-down and Czochralski methods. The crystals were chemically uniform along the growth axis, and their composition was equal to that of the melt, thus corresponding to vicinity of congruent melting composition. Spatial distribution of Ce in La-GPS was also inspected, and no variation of Ce content was detected as a result of its similarity to one of the host cations (La) regarding the size. Basic optical and scintillation properties of the Ce:La-GPS crystals are also reported, and it is demonstrated that partial substitution of Gd with La has no negative impact on crystal growth and physical performance of the crystals. melts incongruently at 1720 °C with decomposition to orthosilicate Gd4(SiO4)3 and the liquid phase. For that reason, simple solidification of Gd2Si2O7 stoichiometric melt results in formation of a nondesired Gd4(SiO4)3 phase that melts congruently. A recent report8 demonstrated that partial substitution of Gd3+ with other large rare-earth cations of La3+ (10 atom %) also stabilizes formation of the Gd2Si2O7 pyrosilicate phase. Such a matrix of mixed (La,Gd)2Si2O7 pyrosilicate host crystal can also be substituted with some amount of active Ce-dopant, and the Ce content in the (La,Gd)2Si2O7 crystal can be as low as necessary. If the Ce content is low enough, then concentration quenching is not observed. Growth of the (La0:09Ce0:01Gd0:90)2Si2O7 crystals by the floating zone (FZ) method was reported in refs 13 and 14. However, the optimal La/Gd ratio for the (La,Gd)2Si2O7 regarding its melting and/or solidification performance was not examined in detail. The goal of the current project was to optimize composition of the Ce:(La,Gd)2Si2O7 mixed crystal based on crystal chemistry of this material and to understand mechanisms responsible for formation of such crystals. Thus, this study is derived from the fusion between crystal growth and materials design. The methods involved in characterization of the LaGPS included crystal growth by micro-pulling-down (μPD)15−17 and Czochralski (CZ) techniques,18 structure identification with X-ray diffraction (XRD), study of the

1. INTRODUCTION A number of halide and oxide scintillator materials have been developed in the past few decades.1−5 Currently, these materials are widely used in various fields including astronomy, medical imaging, and homeland security. Although halide scintillators, such as Tl:NaI, Tl:CsI, and Ce:LaBr3, have high light outputs of more than 30 000 photons/MeV, they are rather hygroscopic which makes their device application comparatively complicated. On the other hand, most oxide scintillators are very resistant to moisture and humidity. Ce-doped gadolinium pyrosilicate (or disilicate), Ce:Gd2Si2O7 (Ce:GPS) crystals also have high light output of 30 000 photons/MeV and FWHM energy resolution of 6.0% at 662 keV at room temperature. However, Ce:GPS melts incongruently. Therefore, it has to be grown from the solution (flux). As an example, Ce:GPS crystals were grown from the melt heavily doped with Ce (approximately 10 atom % regarding Gd+ host cations to be substituted) in order to modify the phase diagram and to stabilize the crystal growth process.6,7 Such excessive Ce concentration leads to reduced light output because of self-absorption or concentration quenching. On the other hand, optimal Ce3+ content with respect to the amount of host rare-earth cations is approximately 1 atom % according to ref 8 or 0.75−2.50 atom % according to ref 9. Unfortunately, this amount of Ce3+ is not sufficient to improve stability of the GPS formation. Some of the other disilicate crystals formed by small rareearth metals including Lu2Si2O7, Yb2Si2O7, and Er2Si2O7 melt congruently.8,10,11 However, undoped Gd2Si2O7 cannot be produced from the melt of its stoichiometric composition.12 It © XXXX American Chemical Society

Received: September 21, 2014 Revised: February 28, 2015

A

DOI: 10.1021/cg501416u Cryst. Growth Des. XXXX, XXX, XXX−XXX

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Crystal Growth & Design

view of the meniscus observed in the steady state of the growth of the Ce:La-GPS crystal from the (La0:98Ce0:02Gd1.00)2Si2O7 melt is illustrated in Figure 2.

crystals compositions and segregation phenomenon, measurements of basic optical and scintillation properties of the materials, and others.

2. CRYSTAL GROWTH 2.1. Micro-Pulling-Down (μ-PD) Method. The starting materials were prepared from the oxide powders of La2O3, CeO2, Gd2O3, and SiO2. Chemical purity of the starting oxides was better than 99.99%. The melt compositions used for the μPD growth are summarized in Table 1. The crystals were grown Table 1. μ-PD Growth Conditions and the Growth Results melt composition

seed

atmosphere

growth results

Gd2Si2O7 (La0:2Gd1.8)Si2O7 (La0:3Gd1.7)Si2O7 (La0:3Gd1.7)Si2O7 (La0:6Gd1.4)Si2O7 (La1.0Gd1.0)Si2O7 (La0:98Ce0:02Gd1.00)2Si2O7

Ir Ir Ir YAG Ir Ir Ir

N2 N2 Ar Ar N2 N2 N2

polycrystal polycrystal polycrystal polycrystal partly transparent transparent, cracks single crystal

Figure 2. View of the meniscus zone in μ-PD growth as observed via CCD camera coupled with a monitor for the growth from the (La0:98Ce0:02Gd1.00)2Si2O7 melt. The crystal diameter is about 5 mm.

2.2. Czochralski (CZ) Method. The Czochralski growth of (La,Gd)2Si2O7 type crystals was also examined. The growths were performed from an Ir crucible of Ø50 × 50 mm in dimensions that was surrounded with zirconia and alumina ceramics for thermal insulation. The crucible was heated inductively. The pulling-up rate applied was 0.5 mm/h, and the environmental atmosphere installed in the growth chamber was Ar + O2 (2%). The seed crystal was rotated at 10 rpm, and the crystal diameter was controlled to be about 25 mm which corresponded to a solidification rate of approximately 1−2 g/h (mass of solidified material per unit of time). At earlier stages, the growth processes were performed from nonoptimized melts like (La0.3Gd1.7)Si2O7. In such conditions, the as-produced solids were highly polycrystalline substances without any single-crystalline grains distinguishable with naked eyes. However, lately when the melts contained about equal fractions of La2O3 and Gd2O3, the solid substances obtained had greater density and contained uniform and transparent single-crystalline fragments that were very noticeable visually. These fragments were then separated from the as-solidified materials and used for preliminary characterization. These crystals were several millimeters in all three dimensions. Thus, fabrication of the specimens with dimensions sufficient for evaluation of their properties became possible. After optimization of the growth parameters, the well-shaped La-GPS crystal was produced using the seed oriented along the ⟨100⟩ direction at a rotation rate of 12 rpm. The Ce:La-GPS crystal grown from the (La0:98Ce0:02Gd1.00)2Si2O7 melt is demonstrated in Figure 3. 35% of the melt was solidified into the crystal, and two specimens were cut from it perpendicular to the growth axis for following inspection of their optical and scintillating performance. The specimens were cut from the fragments of the crystal corresponding to approximately 12% and 27% solidification fraction (Figure 3). These plate-like samples had dimensions of 5 × 5 × 1 mm. As a first step, composition of these specimens was evaluated using scanning electron microscopy combined with electronprobe microanalysis (SEM/EPMA). The scanning of the samples was performed along their large surfaces (i.e., perpendicular to the growth axis). The as-recorded results are illustrated in Figure 4. Surprisingly, no noticeable difference in chemical composition of both samples was detected. Both demonstrated high similarity, and no fluctuations in distribution of rare-earth cations were observed in the radial direction either. Thus, at least in the fragments evaluated, the crystal was exceptionally uniform in all three dimensions. This demon-

using conventional μ-PD systems (two different growth machines) with induction heating.15−17 The growths were performed from Ir crucible of 22 mm in outer diameter with the die of 5 mm in diameter. The growth unit was equipped with a 20 mm long Ir after-heater. The pulling-down rate was 0.1 mm/min. Some growth conditions and the growth results are summarized in Table 1 and Figure 1.

Figure 1. View of the La-GPS crystals produced by the μ-PD method from the melts of (La0.2Gd1.8)Si2O7 (polycrystal, top), (La1.0Gd1.0)Si2O7 (middle), and (La0:98Ce0:02Gd1.00)2Si2O7 (bottom). Details of the growths are available from Table 1. Scales are in mm.

The growths performed from the melts containing approximately equal fractions of Gd3+ and La3+ demonstrated relatively stable solidification behavior during the entire process. This is usually observed when the material melts congruently, and the ratio between its constituents corresponds to the vicinity of congruently melting composition. Typical B


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Figure 3. View of the CZ grown La-GPS crystal produced from the melts of (La0:98Ce0:02Gd1.00)2Si2O7 composition (bottom), and specimens (top) cut from the crystal at positions corresponding to different solidification fractions. Scale is in mm.

Figure 5. XRD data for the samples A and B cut from the CZ grown La-GPS crystal produced from the melts of (La0:98Ce0:02Gd1.00)2Si2O7 composition. The samples A (top) and B (middle) were cut from the crystal according to Figure 3. The XRD data (bottom) are from ref 6 for the Ce:Gd2Si2O7 crystal with triclinic structure. These results are also coincident with those reported in ref 8 for triclinic Ce0:2Gd1.8Si2O7.

3. PHASE DIAGRAM OF (La,Gd)2Si2O7 Considering the phase diagrams reported in the past,10,12,19,20 both end members of the La-GPS mixed crystal system (La2Si2O7 and Gd2Si2O7) melt incongruently with decomposition temperatures of 1750 and 1720 °C, respectively.19 The phase diagrams of both these compounds are illustrated in Figure 6. It is important to note that in the case of La2Si2O7, the phase that is formed in the melt of La2Si2O7 instead of corresponding La 2Si 2O7 pyrosilicate is 7La 2O3 × 9SiO2 (La14Si9O39) that has hexagonal apatite structure.22 However, in the melt of Gd2Si2O7, the first solidified phase is Gd4(SiO4)3 orthosilicate. Unfortunately, the phase diagram of the pseudobinary system La2Si2O7−Gd2Si2O7 was not reported. Therefore, the actual behavior of the La2Si2O7−Gd2Si2O7 solid solutions cannot be confirmed. However, the phase diagrams of the prototype pseudobinary systems of La2Si2O7−Y2Si2O7 and La2Si2O7− Yb2Si2O7 were analyzed in the past.10,23−25 Both of these systems (Figure 7) demonstrated existence of invariant points for the mixed (La,Y)2Si2O7 and (La,Yb)2Si2O7 compounds of certain compositions. These could be considered as congruently melting compositions (junction of solidus and liquidus lines on the phase diagrams). In first case of (La,Y)2Si2O7 (Figure 7, above), such a compound is formed as a result of admixing of two incongruently melting compounds. However, in the case of (La,Yb)2Si2O7 (Figure 7, below), such a compound is formed from incongruently melting La2Si2O7 and

Figure 4. Distribution of the constituents in the CZ grown La-GPS crystal produced from the melt of (La 0:98 Ce 0:02 Gd 1.00 ) 2 Si 2 O 7 composition. The samples A (top) and B (bottom) were cut from the crystal according to Figure 3.

strated that no segregation occurred when the crystal was produced from the (La0:98Ce0:02Gd1.00)2Si2O7 melt. At first approximation, this observation confirmed that the (La0:98Ce0:02Gd1.00)2Si2O7 melt was not very distant from congruently melting composition. Powder X-ray diffraction (XRD) analysis was also used to perform phase identification of the crystal produced from the (La 0:98Ce0:02Gd1.00) 2Si2O7 melt (Figure 5). The results indicated that the structure of the Ce:La-GPS crystal was identical to that of Ce:GPS,6 and it was triclinic The ratios of the intensities of the XRD peaks detected for the A and B specimens (Figures 3−5) were somewhat different from each other and that of Ce:GPS.6 However, positions of the peaks fitted well enough. C


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Figure 7. Phase diagrams of the La2Si2O7−Y2Si2O7 (top) and La2Si2O7−Yb2Si2O7 (bottom) systems reproduced from refs 10, 23 and 10 as is, respectively. Compositions are presented in wt %. Notice the existence of invariant points (at minimum of the liquidus lines) for the mixed (La,Y)2Si2O7 and (La,Yb)2Si2O7 compositions. Figures reprinted with permission from ref 10. Copyright 1982 Elsevier.

ref 9, the structure of the (Ce,Gd)2Si2O7 compounds changes as a result of replacement of Gd with Ce; substitutions with 0− 10 atom %, 10−60 atom %, or 60−100 atom % Ce result in the formation of orthorhombic, triclinic, or monoclinic phases, respectively. This is illustrated in Figure 8. A similar trend was observed29 when intermediate Gd3+ cation was substituted with another large rare-earth cation of Pr3+. It was also noted11 that undoped Gd2Si2O7 may exist in two modifications (orthorhombic E- and triclinic B-types). On the other hand, growth of La0.2Ce0.02Gd1.78Si2O7 pyrosilicate mixed crystals with tetragonal structure was reported in refs 8 and 30, and these crystals demonstrated one of the highest luminescence yields. Thus, evolution of the structure of the Gd2Si2O7 end member with partial substitution of Gd3+ host cation with larger guest cations of La3+, Ce3+, and/or Pr3+27 may follow two routes. In one case, the structure transforms from orthorhombic to triclinic. Triclinic structure (B-type) is sometimes referred as the αphase28 for the RE2Si2O7 pyrosilicates (Figure 8, Table 2). In another case, the structure has tetragonal symmetry at least for low concentrations (10 atom %) of La. These conclusions made based on experimental data of refs 8 and 9 are also summarized in Figure 8. It is noted that formation of different structures in the Gd2Si2O7 crystals (tetragonal vs orthorhombic ones) was also associated with different growth temperatures.8 The samples A and B cut from the La-GPS crystal produced in this work from the melt of (La0:98Ce0:02Gd1.00)2Si2O7 composition (Figures 3 and 4) had triclinic structure (Figure 5) with space group P/1.6 This result is in good agreement with Figure 8 assuming that enrichment of the Gd2Si2O7 end member with La and Ce has a comparable effect on the phase transformations (note similar dimensions of La and Ce27).

Figure 6. Phase diagrams of La2O3-SiO2 (top) and Gd2O3-SiO2 (bottom) systems according to refs 10 and 21 and ref 12, respectively. Both La2Si2O7 and Gd2Si2O7 melt incongruently. Panel (top) reprinted with permission from ref 10 (Copyright 1982 Elsevier). Panel (bottom) reprinted from ref 12 with kind permission from Springer Science and Business Media.

congruently melting Yb2Si2O7. The latest diagram is practically identical to that proposed in ref 26. Also, ionic radii of R8(Y3+) = 101.9 pm and R8(Gd3+) = 105.3 pm determined for coordination number CN = 8 are relatively close with difference of about 3%.27 These values are much less than that of R8(La3+) = 116 pm. Taking into consideration the similar chemical nature of all rare-earth metals, assumption about the similarity of the La2Si2O7−Y2Si2O7 and La2Si2O7− Gd2Si2O7 phase diagrams seems to be acceptable. Therefore, the reported La2Si2O7−Y2Si2O7 diagram is illustrated in Figure 7 also to assist discussion on behavior of (La,Gd)2Si2O7 mixed crystals below.

4. DISCUSSION 4.1. Structure of Ce-Doped (La,Gd)2Si2O7). Formation of up to seven different crystallographic structures in the rareearth (RE) disilicates with general formula of RE2Si2O7 was reported in the past.28 The crystalline structure of mixed (La,Ce,Gd)2Si2O7 compounds depends on the amount of La3+ and/or Ce3+ cations introduced into the crystal. According to D


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the Si4+ cations in all these structures are not considered in these speculations as variable parameters because these cations were not substituted in all the above growth experiments. Also, formation of any antisite defects based on substitution of Si4+ with any rare-earth cation or opposite is practically impossible. This follows from the large size difference (2−3 times regarding cation radii27) between silicon and rare-earths. However, all three structures of interest (including tetragonal, triclinic, and orthorhombic ones, Figure 8) can be modified and sometimes stabilized through redistribution of the RE-cations within different RE-sites (or RE-sublattices). The orthorhombic structure has only one type of RE-sites according to ref 22 and Table 2. Therefore, cations are distributed between all these equivalent RE-sites randomly. On the other hand, both tetragonal and triclinic structures contain four nonequivalent crystallographic positions32−36 creating additional variables. These variables are associated with intersite segregation and nonuniform distribution of different RE cations between different RE-sites that may affect the structure behavior and its stability. Different bond distances observed in pyrosilicates are responsible for different volumes of the RE-sites V[RE]. Therefore, nonuniform distribution of RE-cations having different ionic radii is observed in the structures containing more than one type of RE-site.24,38 Thus, preferable occupation of some RE-sites with a certain type of RE-cation is observed when mixed crystals are formed. On the basis of the existence of four nonequivalent RE-sites in triclinic Tm2Si2O7, its chemical formula was proposed in ref 35 to be Tm4[Si3O10][SiO4]. Following this suggestion, a similar formula of [RE1][RE2][RE3][RE4]Si4O14 could be also considered as a helpful interpretation tool suitable for understanding behavior of the (La,Ce,Gd)2Si2O7 mixed crystals. Notice that RE1, RE2, RE3, and RE4 correspond to rare-earths residing in four different crystallographic sites having different coordination polyhedrons with different volumes (Figure 9). In

Figure 8. Evolution of crystalline structure of Gd2Si2O7 after substitution of Gd3+ with larger rare-earth cations of La3+, Ce3+, and/or Pr3+ according to refs 8, 9, and 29−31. The RE2Si2O7 pyrosilicate (or disilicate) compounds form four types of structures including monoclinic (1 as shown on vertical axis), triclinic (2), orthorhombic (3), and tetragonal (4) ones. Evolution of structure of Gd2Si2O7 with substitution of Gd3+ with large cations follows stepped curve marked with open circles (in direction of the arrow) from point with coordinates (3, 0%) to point (1, 100%). Structure type 4 is illustrated for reference. References are shown in square brackets.

Formation of triclinic structure is observed when the growth is performed by the CZ method from the appropriate stoichiometric RE2Si2O7 melt not containing any excess of SiO2. Oppositely, the presence of excess of SiO2 transformed melt growth to self-flux growth process at reduced growth temperatures.8 4.2. Segregation of Rare-Earth Cations in RE2Si2O7 Pyrosilicates. The crystals discussed here contained several RE cations. All of them demonstrate a very similar chemical nature differing in dimensions mostly. Therefore, it is always important to understand distribution of such cations inside the structure and to evaluate the effect of cationic radii of RE constituents27 on formation and stability of the product crystalline phase. This consideration was applied in the past for a number of complex oxide crystals including garnets and others.26,37 Below, similar argumentation is applied to explain experimental results observed in the melt growth of (La,Ce,Gd)2Si2O7 mixed crystals. Table 2 summarizes some properties of the three structural types detected in the pyrosilicates/disilicates within the compositional range discussed here (Figure 8). Positions of

Figure 9. Structural motive of [RE1][RE2][RE3][RE4]Si4O14 ideal quasi-quaternary compound with four different RE-cation sites constructed similarly to refs 26 and 37 for V[RE1] > V[RE2] > V[RE3] > V[RE4] and invariant Si-sites.

fact, the purpose of “invention” of [RE1][RE2][RE3][RE4]Si4O14 formula cannot be considered as the desire to make

Table 2. Selected Structural Properties of [RE1][RE2][RE3][RE4]Si4O14 Pyrosilicate (Disilicate) Compounds with Tetragonal, Triclinic, and Orthorhombic Structuresa structure

type

tetragonal triclinic orthorhombic

A B E

type

unit cell

RE-sites and their CN

refs

α δ

a = b ≠ c, 90° a ≠ b ≠ c, ≠90° a ≠ b ≠ c, 90°

RE1: CN = 7, RE2: CN = 9, RE3: CN = 8(7), RE4: CN = 9 RE1: CN = 8(7 + 1), RE2: CN = 8, RE3: CN = 6, RE4: CN = 8 RE1 = RE2 = RE3 = RE4

32−34 34−36 24

a

CN is the coordination number of RE-sites. Note that tetragonal and triclinic structures have four types of non-equivalent RE-sites, and their effective volumes are generally different V[RE1] ≠ V[RE2] ≠ V[RE3] ≠ V[RE4]. CN and shapes of coordination polyhedrons vary depending on reference source. E


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Crystal Growth & Design simple things complicated. As an example of similar practice, authors reference to well-known Y3Al5O12 (YAG) crystal. Crystallographically correct and commonly accepted formula of the YAG crystal is {Y}3[Al2](Al3)O12 (see for example,16 p 101), where cations enclosed in { }, [ ], and () correspond to those occupied dodecahedral, octahedral, and tetrahedral sites, respectively. 4.3. Segregation and Phase Stability of (La,Ce,Gd)2Si2O7. For the further discussion on the segregation and phase stability of the (La,Ce,Gd)2Si2O7 mixed crystals, we define the crystal structure as a matrix of oxygen anions and positively charged cations that are surrounded by oxygen anions and ordered in a specific arrangement. Electronegative oxygen anions are nonsubstitutable, they are sited in fixed positions, and they tend to gain some electrons from positively charged cations of RE3+ (La3+, Ce3+, and Gd3+) and Si4+.39 Positively charged cations are bonded to oxygen anions as a result of electrostatic attraction between positive and negative ions and overlap of two electron clouds of neighbor ions. The positions of the cations in the structure are also fixed; however, they may be substituted with other positively charged cations if their dimensions are compatible and charge balance is satisfied. Details of the anion and cation positions and their coordination numbers are available for tetragonal structure in refs 32−34 for triclinic structure in refs 34−36, and for orthorhombic structure in ref 24. In the above (La,Ce,Gd)2Si2O7 crystal growth experiments, the mixed crystal products had generally better quality than those of the corresponding end members, and all of the mixed crystals had triclinic structure. Therefore, hypothetical interpretation of this result is proposed according to illustrations given in Figures 9−11. Existence of four geometrically nonequivalent RE sites assumes nonuniform distribution of

Figure 11. Structural motives of unstable and stable [RE1][RE2][RE3][RE4]Si4O14 quasi-binary compounds with two different type RE-cation sites constructed similarly to refs 26 and 37 for V[RE1] ≈ V[RE2] > V[RE3] ≈ V[RE4] and invariant Si-sites.

nonequivalent (regarding the ionic radii 27) RE-cations following geometrical and dimensional criteria. This is a common tendency observed in many oxide crystals.26,37 As a rule, large cations tend to occupy large crystallographic sites and opposite. Finally, most stable oxide structures are formed when RE-cations reside in the sites that fit their dimensions in the best way. Such highly stable oxide compounds have expanded crystallization fields on the phase diagrams. Sometimes, substitution of “unstable” cations with “stable” ones transforms incongruently melting materials to congruently melting ones. Also, formation of such crystals including initial nucleation and single crystal growth proceeds much more easily than formation of alternative unstable isomorphs with nonoptimized composition (Figures 10 and 11). Regarding [RE1][RE2][RE3][RE4]Si4O14, actual volumes (V) of the different RE-sites cannot be well determined as the volumes of simple geometrical polyhedrons. Their ability to accommodate different RE-cations depends not only on oxygen anions forming first coordination sphere, but also on the arrangement of other distant particles that appear in second and even third coordination spheres surrounding the cation. Thus, effective volumes of the RE-sites do not always follow their geometrical volumes resulting in a not always predictable substitution mechanism. Moreover, action of other driving forces supporting incorporation of any particular RE-cation into any particular RE-site except geometrical tolerance (“radius crirerion” rule38) should be also considered32 from the point of view of minimization of the total energy of the crystalline structure. Generally, effective volumes of the different type RE-sites are not equal (V[RE1] ≠ V[RE2] ≠ V[RE3] ≠ V[RE4]) for both the tetragonal and triclinic structures. However, volumes of some of such RE-sites can be similar regarding their ability to accept certain types of RE-cations in almost equal amount.

Figure 10. Structural motives of unstable and stable [RE1][RE2][RE3][RE4]Si4O14 quasi-binary compounds with two different type RE-cation sites constructed similarly to refs 26 and 37 for V[RE1] > V[RE2] ≈ V[RE3] ≈ V[RE4] and invariant Si-sites. F


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Authors note that the model of formation of pyrosilicate mixed crystals introduced above is generally hypothetical one regarding this type of structure. Therefore, following detailed research on performance of all RE-sites in pyrosilicates is necessary. However, a similar model worked reasonably well for explanation of behavior of other complex oxide materials.26,37 As an example, consideration of two types of tetragonal sites (having different effective volumes) in well-known langasite type structures allowed the discovery of new fully ordered crystals of the langasite family back in 2000.40 For examination of the above model, the growth of the La-GPS crystal was also performed from the melt of (La 0.485 Gd 1.5 Ce 0.015 )Si 2 O 7 composition. The as-produced 2 in. diameter crystal (Figure 13) had neither cracks nor visible inclusions.

Many combinations could be considered based on various relationships between dimensions of various RE-sites (Figure 9). Only two such example combinations of “dimensions” of different RE-sites are illustrated in Figures 10 and 11 to reduce volume of this report. These two combinations are associated with V[RE1] > V[RE2] ≈ V[RE3] ≈ V[RE4] and V[RE1] ≈ V[RE2] > V[RE3] ≈ V[RE4] relationships. Thus, for the [RE1][RE2][RE3][RE4]Si4O14 compounds, containing two types of RE, the optimal ratio resulting in the formation of the highly stable structure could be approximately 1:3 or 1:1. This is correct only for the two selected cases illustrated in the Figures 10 and 11, respectively. These ratios (1:3 and 1:1) somehow correspond to optimal La/Gd ratios of 1:3 and 1:1 observed in the (La,Ce,Gd)2Si2O7 growths performed by CZ and μ-PD methods. Two above simple ratios correspond to the ideal case when 100% of large cations are sited in the large RE-sites and 100% of relatively small cations are sited in small RE-sites (Figures 10 and 11). This did not happen in the real world because of statistically possible antisite substitution associated with partial incorporation of small RE into large RE-sites and opposite. This substitution is probably negligible, but it is certainly unavoidable even when difference between cation radii is high. The difference between the small (Gd3+) and large (La3+) cations discussed here is less than 10% regarding cation radii27 which is noticeable but not an exceptionally high value. Therefore, some antisite RE substitution is evidently possible. In addition, formation of the ideal nonstressed structures illustrated in middle structural motives of Figures 10 and 11 is not possible because of the limited number of types of cations available in nature. As a result, the minimum energy state of the crystal regarding any individual RE-sublattice can be achieved through admixing of different types of cations that not necessarily have same dimensions as those of the available RE-sites. This is demonstrated schematically in Figure 12.

Figure 13. View of the CZ grown 2 in. diameter crystal of La-GPS produced from the melts of (La0.485Gd1.5Ce0.015)Si2O7 composition. Neither cracks nor visible inclusions are observed. Scale is in mm.

5. OPTICAL AND SCINTILLATION PROPERTIES Optical transmission spectra were recorded in the wavelength range of 200−900 nm using a V550 spectrophotometer produced by JASCO. The results are illustrated in Figure 14. Both A and B samples (Figure 3) demonstrated practically identical performance regarding their absorption. The position of the absorption edge of the A and B specimens was similar to that reported for the La0.09Ce0.01Gd0.90Si2O7 crystal produced by FZ method in ref 13. However, transparency of the CZ grown crystal discussed here was somewhat better than that of the FZ grown one. This observation could be associated with (I) a different level of La-substitution and (II) better homogeneity of the melt in the CZ process as compared to FZ growth. An additional reason (III) could be the result of better structural stability of the La-enriched La-GPS crystals grown from the melt of (La0:98Ce0:02Gd1.00)2Si2O7 composition (A and B samples) again compared to those grown from the La0.09Ce0.01Gd0.90Si2O7 melt13 as it was discussed in the previous section. Thus, formation of a more structurally stable compound enriched with La3+ is favorable resulting in lower concentration of comparatively unfavorable defects. The photoluminescence measurements were also performed in the same range using the spectrofluorometer FLS920 (Edinburgh Instruments) equipped with the hydrogen steadystate, nanosecond, and Xe microsecond pulsed flash-lamps (IBH Scotland). Fragment of photoluminescence spectra

Figure 12. Structural motives of selected ideal (above) and actual RE2-sublattice consisted of crystallographically identical RE2-sites when ideal RE-B cations are not available in the system (below).

The considerations presented above for the [RE1][RE2][RE3][RE4]Si4O14 pyrosilicate compounds with triclinic structure are also applicable to tetragonal pyrosilicates because they are also constructed from four nonequivalent RE-sites offered for accommodation of different RE cations (Table 2). Certainly, RE-sites of triclinic pyrosilicates are not equivalent to those of tetragonal pyrosilicates. This is a result of a different arrangement of the particles in the structures of different symmetry. G


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Figure 16. Radio-luminescence spectra of the A and B samples cut according to Figure 3. The spectra were recorded under excitation of the samples with 5.5 MeV alpha-rays emitted from the 241Am source.

10% La-substituted crystals grown by the FZ method previously13 were approximately the same. For the scintillation decay measurements, the crystals were covered with several layers of Teflon tape and attached to the surface of the 137Cs isotope source. The other side of the samples was optically coupled to the light entrance window of the photomultiplier tube (PMT) R7600 (Hamamatsu) with silicone optical grease of OKEN 6262A. This way, the 662 keV γ-photons emitted from the 137Cs isotope excited the 1 mm thick samples resulting in emission of scintillation photons with energies corresponding to the visible wavelength range. The high voltage was supplied to PMT from the ORTEC 556 source. The signals were read out from the anode of the PMT. Immediately after, the signals passed through an preamplifier, and then they were sent to the shaping amplifier ORTEC 570 to convert originally very sharp signals with very long tails to more symmetrical near-Gaussian shaped pulses. This was necessary to improve the signal-to-noise ratio. The pulses were also amplified at this stage for following amplitude categorization. Shaping time of the amplifier was 0.5 μs. Finally, the pulses were converted to digital signals, sorted according to their amplitudes, and accumulated in corresponding bines (channels) by a multichannel analyzer Pocket MCA 8000A provided by Amptek Co. These signals were then transferred to a personal computer for further analysis. Decay time was evaluated with the help of an oscilloscope, Tektronix TDS3052B. The output signal from PMT was supplied directly to the oscilloscope. The example scintillation decay time profile measured for the sample A is illustrated in Figure 17. Similar results were obtained for the sample B. The observed

Figure 14. Transmission spectra of the A and B samples (top) cut according to Figure 3 and the spectrum of the La0.09Ce0.01Gd0.90Si2O7 crystal grown by FZ method in ref 13 (bottom).

recorded in the range of 200−500 nm is illustrated in Figure 15. Once again, no detectable differences between two spectra

Figure 15. Photoluminescence spectra of the sample B cut according to Figure 3 measured under excitation with Xe-lamp. Similar spectra was obtained from sample A.

corresponding to specimens A and B were detected demonstrating uniformity of the crystal produced from the melt of (La0:98Ce0:02Gd1.00)2Si2O7 composition. For the radio-luminescence spectra measurements, the specimens were excited with 5.5 MeV alpha-rays produced by 241 Am source. The spectra were recorded with the same spectrofluorometer, and the results are demonstrated in Figure 16. Also, no difference between the spectra of A and B samples was detected confirming uniformity of the discussed crystal. It is noticed that positions of the emission peaks in both photoand radio-luminescence spectra of the CZ grown La-GPS and

Figure 17. Scintillation decay time profile of the sample A cut according to Figure 3. H


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properties. The boundaries of these ranges correspond to the crystals illustrated in Figures 3 and 13. The above conclusions demonstrated that partial substitution of Gd with La in La-GPS crystals transforms originally incongruently melting compound to congruently melting one without remarkable negative factors. Ce-doped gadolinium pyrosilicates are expected to be used for the well logging applications that require 2- or 3-in. diameter bulk single crystals. Therefore, the crystal design considerations used to understand congruent melting of La-GPS mixed crystals is one of the most important points of this report. In spite of significant progress in understanding and practical development of La-GPS crystals, future efforts are necessary to optimize the crystal composition, growth conditions, and scintillation properties.

scintillation decay profiles were well approximated with double exponential fitting. The magnitudes of the detected components were τ(A-fast) = 65 ns (87%), τ(A-slow) = 685 ns (13%), τ(B-fast) = 63 ns (86%), and τ(B-slow) = 670 ns (14%). Thus, these parameters were also identical for both A and B specimens (Figure 3), also confirming that uniformity of the crystal grown from the (La0:98Ce0:02Gd1.00)2Si2O7 melt was exceptionally high. Improvement of scintillating performance of the mixed crystals as compared with the performance of the end member crystals was recently observed in number of alkali-earth halides and oxides.41−43 Most of these materials demonstrated a tendency to have highest scintillation intensity at maximum admixing when the end member compounds are blended in the matrix in about equal molecular amounts.43 This improvement was associated with inhomogeneous local distribution of the cations that affected bottom of the conducting band. Most of these solid solution materials had the same structure in the full range of concentrations including both end members and mixed crystals formed within this range. However, the Ce:LaGPS single crystals discussed here behave differently regarding their phase formation; both end members do not melt congruently. Nevertheless, such end members participate in the formation of the mixed Ce:La-GPS crystals in a limited range from 10 to 60 atom % regarding La content (Figure 8). Therefore, scintillation properties of the Ce:La-GPS crystals are supposed to be similar to those mixed crystals formed by congruently melting end members noticed in refs 41−43.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is partially supported by (i) Adaptable & Seamless Technology Transfer Program through Target-driven R&D (ASTEP), JST (ii) Japan Society for the Promotion of Science (JSPS) Grant-in-Aid for Exploratory Research (AY), (iii) Development of Systems and Technology for Advanced Measurement and Analysis, Japan Science and Technology Agency (JST), (iv) the funding program for next generation world-leading researchers, JSPS. In addition, we would like to thank following for their support: Mr. Hiroshi Uemura, Ms. Keiko Toguchi, Ms. Megumi Sasaki, and Ms. Yuka Takeda in IMR. The authors thank Mr. Sugawara, Ms. Nomura at Crystal Growth & Design, Cooperative Research and Development Center for Advanced Materials, IMR, Tohoku University.

6. SUMMARY Growth of (La,Gd)2Si2O7 (La-GPS) mixed crystals was established using two alternative melt growth techniques (micro-pulling-down and Czochralski methods). The results demonstrated that independently of the growth technique applied, formation of these mixed crystals is possible in spite the fact that both end members of the La-GPS system (La2Si2O7 and Gd2Si2O7) melt incongruently. This result is explained based on a description of the structure of triclinic pyrosilicate crystals having four nonequivalent rare-earth sites. It is assumed that nonuniform distribution of La3+ and Gd3+ between crystallographically nonequivalent RE-sites is the main reason why (La,Gd)2Si2O7 mixed crystal is more competitive than La2Si2O7 and Gd2Si2O7 end members. The La-enriched La-GPS crystals grown from the melt of (La0:98Ce0:02Gd1.00)2Si2O7 composition (A and B samples, Figure 3) were single phase materials of high chemical homogeny along growth axis and radial directions. In addition, these materials demonstrated exceptional repeatability of their optical (transmission spectra) and scintillation properties including photo- and radio-luminescence spectra, and 137Cs gamma-ray excited scintillation decay time profiles. According to ref 9, the range of Ce substitution necessary for stabilization of triclinic crystalline structure of (Ce,Gd)2Si2O7 is 10−60 atom %. However, this amount of Ce is unacceptably high for appropriate functioning of the (Ce,Gd)2Si2O7 crystals in actual scintillating applications. Therefore, in the (La,Ce,Gd)2Si2O7 crystals reported here, the triclinic structure was stabilized with a considerable amount of La. Additionally, a small amount of Ce was used to establish suitable scintillating properties. Thus, according to our results, 25−50 atom % of La is sufficient to stabilize the triclinic structure, and 0.75−1.00 atom % of Ce is sufficient to demonstrate proper scintillating

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REFERENCES

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