Differences and Similarities between Lanthanum and Rare-Earth

Article pubs.acs.org/IC

Differences and Similarities between Lanthanum and Rare-Earth Iodate Anhydrous Polymorphs: Structures, Thermal Behaviors, and Luminescent Properties Yan Suffren,†,‡,§ Olivier Leynaud,†,‡ Philippe Plaindoux,†,‡ Alain Brenier,⊥ and Isabelle Gautier-Luneau*,†,‡ †

Université Grenoble Alpes, Institut NEEL, F-38042 Grenoble, France CNRS, Institut NEEL, F-38042 Grenoble, France § INSA, UMR 6226, Institut des Sciences Chimiques de Rennes, 20 Avenue des buttes de Coësmes, F-35708 Rennes, France ⊥ Univ Lyon, Université Claude Bernard Lyon 1, CNRS, Institut Lumière Matière, F-69622, Lyon, France ‡

S Supporting Information *

ABSTRACT: The Ln(IO3)3(HIO3)y (y = 1 or 1.33) compounds are isostructural with the La(IO3)3(HIO3)y phases, but thermal studies reveal different behaviors. On the one hand, the partial thermal decompositions of these lanthanide compounds lead to the Ln(IO3)3 formulation, with a room temperature structure different from the β-La(IO3)3 obtained from La(IO3)3(HIO3)y. On the other hand, the partial thermal decompositions of the La1−xLnx(IO3)3(HIO3)y compounds prepared with lanthanides ions (Ce, Pr, Nd, Sm, Eu, Gd, and Yb) lead to acentric β-La1−xLnx(IO3)3. As for β-La(IO3)3, reversible structural transitions from β-La1−xLnx(IO3)3 to centrosymmetric γ-La1−xLnx(IO3)3 are observed. Differential scanning calorimetry analyses of La1−xLnx(IO3)3 solid solutions show that the transition temperatures vary with the lanthanide concentration in the solid solution. A transition is observed only up to a certain fraction of lanthanide-ion substitution; this substitution limit decreases with the cationic radius of the lanthanide ion. Finally, the βLa1−xNdx(IO3)3 and β-La1−xYbx(IO3)3 phases are investigated by luminescence spectroscopy.

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(IO3)4,15 and Nd3+, Yb3+-doped Y(IO3)3 have already been observed.16 Unfortunately, both yttrium iodate structures are centrosymmetric.17 Recently, we have presented the structures and thermal behaviors of six lanthanum iodates: La(IO3)3(HIO3), La(IO3)3(HIO3)1.33, and α-, β-, γ-, and δ-La(IO3)3.18−20 All of the determined structures present a three-dimensional network of ionocovalent strong bonds, where isolated lanthanum polyhedra are linked by bridging iodate groups. α-La(IO3)3, β-La(IO3)3, and La(IO3)3(HIO3)1.33 possess an acentric structure. La(IO3)3(HIO3) and La(IO3)3(HIO3)1.33 compounds are obtained by slow crystallization in a nitric acid solution (7 M), whereas α-La(IO3)3 is synthesized by a hydrothermal route.18 The thermal decomposition of La(IO3)3(HIO3)y (y = 1 and 1.33) with the loss of 1 or 1.33 molecule of HIO3, respectively, leads to the formation of δ-La(IO3)3, followed by a phase transition into γ-La(IO3)3 and finally into β-La(IO3)3. The latter is stable at room temperature. A reversible phase transition from γ-La(IO3)3 to β-La(IO3)3 occurs at 140 and 185 °C during cooling and heating, respectively (as summarized in

INTRODUCTION

The need for IR frequency converters, working specifically from 0.2 to 12 μm, leads to the engineering of efficient nonlinearoptical (NLO) crystals that shift the light of commercial lasers to specific wavelengths in the atmospheric transparency windows. Another interest is to obtain bifunctional materials able to directly perform second harmonic generation (SHG) from their fluorescence in the visible or in the IR range because more robust and compact solid-state sources could be designed.1,2 The main promising self-doubling materials are crystals based on a nonlinear matrix doped with lanthanide fluorescent ions, such as, for example, Nd3+:YCOB, Nd3+:GAB, Nd3+:YAB, or Yb3+:YAB.3−5 At the present time, only a few hosts are known to be transparent beyond 6 μm. The few laser emissions in the mid-IR range (5−10 μm) are mainly based on the low-phonon-energy hosts, such as the 7.2 μm oscillation of Pr3+ in LaCl3 or the 5.5 μm oscillation of Dy3+ in RbPb2Cl5.6−8 Because most of iodate hosts are transparent from the visible to the beginning of the far-IR range (12 μm),9−13 the doping of these compounds by luminescent lanthanide elements is attractive in the prospect of mid-IR lasers. In our previous studies, the luminescence properties of Cr3+-doped α-In(IO3)3,14 Nd3+,Yb3+-doped AgGd(IO3)4 and Nd3+-doped AgLa© XXXX American Chemical Society

Received: July 30, 2016

A

DOI: 10.1021/acs.inorgchem.6b01850 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

[IO3−]:[Ln3+] molar ratio equal to 5) were dissolved in 40 mL of a nitric acid solution (7 M) at 50 °C. Slow evaporation over 6 days led to the crystallization of colored Ln(IO3)3(HIO3) prisms. Crystals were filtered, washed with water, and dried at room temperature. Synthesis of Ln(IO3)3(HIO3)1.33 with Ln = Ce, Pr, Nd, Sm, and Eu. A total of 0.5 mmol of Ln(NO3)3·nH2O and 5 mmol of KIO3 (with a [IO3−]:[Ln3+] molar ratio equal to 10) were dissolved in 40 mL of a nitric acid solution (7 M) at 50 °C. Evaporation of the solution over 4 days led to the crystallization of colored Ln(IO3)3(HIO3)1.33 rods with a hexagonal cross section. Crystals were filtered, washed with water, and dried at room temperature. Syntheses of La1−xLnx(IO3)3 with Ln = Ce, Pr, Nd, Sm, Eu, Gd, and Yb (with 0.01 ≤ x ≤ 0.85). La1−xLnx(IO3)3 phases were obtained by the thermal decomposition of La1−xLnx(IO3)3(HIO3)y compounds (with y = 1 or 1.33), in an oven at 480 °C, with elimination of HIO 3 molecules. For the syntheses of La1−xLnx(IO3)3(HIO3)y with Ln = Ce, Pr, Nd, Sm, Eu, Gd, and Yb (with 0.01 ≤ x ≤ 0.85), 0.5 mmol of (LaCl3 + Ln(NO3)3·nH2O) and 5 mmol of KIO3 (with a [IO3−]/([La3+] + [Ln3+]) = 10) were dissolved in 40 mL of a nitric acid solution (7 M) at 50 °C. Evaporation of the solution led either to the crystallization of La1−xLnx(IO3)3(HIO3)1.33 rods with a hexagonal cross section or to a mixture with La1−xLnx(IO3)3(HIO3) prisms. Crystals were filtered, washed with water, and dried at room temperature. Elemental analyses of the La1−xLnx(IO3)3 series were performed, demonstrating slight but not uncommon differences between the nominal and final molar fractions in lanthanide ions. Crystal Structures. Single crystals of Pr(IO3)3(HIO3), Pr(IO3)3(HIO3)1.33, Nd(IO3)3(HIO3), and Nd(IO3)3(HIO3)1.33 were mounted on a Nonius κCCD diffractometer, using Ag Kα radiation (λ = 0.56087 Å) at 293 K. The reflections were corrected for Lorentz and polarization effects. An absorption correction was applied using the Gaussian integration method.21 The structures were solved by direct methods with SIR9222 and refined by full-matrix least squares, based on F2 using the SHELXL software23 through the WinGX program suite.24 Final refinements were performed with anisotropic thermal parameters for all of the non-H atoms. The H atoms, not localized in

Figure 1). For the α-La(IO3)3 phase, no transition is observed before decomposition at 470 °C.

Figure 1. Transformation and transition occurring between different anhydrous lanthanum iodate polymorphs.

This study has revealed that γ-La(IO3)3 is isostructural with α-Y(IO3)3 and α-Ln(IO3)3. In this paper, we present the syntheses of new Ln(IO3)3(HIO3)y (y = 1 or 1.33) phases isostructural with La(IO3)3(HIO3)y. The partial thermal decomposition of these lanthanide phases leads to the Ln(IO3)3 formulation, with a structure at room temperature different from β-La(IO3)3. Furthermore, La1−xLnx(IO3)3(HIO3)y-doped phases have been prepared with the following ions: Ce, Pr, Nd, Sm, Eu, Gd, and Yb. Their partial thermal decompositions lead to the La1−xLnx(IO3)3 compounds. The new series of compounds keeps a behavior similar to that of the β-La(IO3)3 host and allows us to study the lanthanide effects. The luminescence spectroscopy of β-La1−xNdx(IO3)3 and βLa1−xYbx(IO3)3 is presented.

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EXPERIMENTAL SECTION

Synthesis of Ln(IO3)3(HIO3) with Ln = Pr, Nd, Sm, and Eu. A total of 0.5 mmol of Ln(NO3)3·nH2O and 2.5 mmol of KIO3 (with a

Table 1. Crystal Data and Structure Refinement Details for Pr(IO3)3(HIO3), Nd(IO3)3(HIO3), Pr(IO3)3(HIO3)1.33, and Nd(IO3)3(HIO3)1.33 formula M (g mol−1) cryst color cryst shape size (mm) cryst syst space group a (Å) b (Å) c (Å) β (deg) V (Å3) Z Dx (g cm−3) μ (mm−1) θ range for data Nhkl,collected Nhkl,unique [I > 2σ(I)] Rint R1 [I > 2σ(I)]a wR2 (all data)b GOF on F2 Δρmax (e Å−3) Δρmin (e Å−3) a

PrI4O12H 841.51 green prism 0.06 × 0.09 × 0.25 monoclinic P21/c (No. 14) 10.634(1) 7.583(1) 14.220(1) 110.57(1) 1073.5(2) 4 5.200 8.524 2.4−21.4 12273 2441 0.046 0.024 0.053 1.174 1.12 −1.05

NdI4O12H 844.84 purple prism 0.02 × 0.05 × 0.14 monoclinic P21/c (No. 14) 10.608(1) 7.551(1) 14.193(1) 110.52(1) 1064.7(2) 4 5.264 8.760 2.4−21.4 18959 2431 0.058 0.032 0.059 1.262 1.26 −1.94

PrI4.33O13H1.33 900.14 green hexagonal rod 0.45 × 0.06 × 0.05 trigonal R3c(H) (No. 161) 22.239(1)

NdI4.33O13H1.33 903.47 purple hexagonal rod 0.16 × 0.06 × 0.06 trigonal R3c(H) (No. 161) 22.212(1)

13.622(1)

13.538(1)

5834.5(6) 18 4.605 7.484 2.5−21.4 20887 2883 0.048 0.037 0.093 1.058 4.17 −4.16

5784.4(6) 18 4.662 7.686 2.9−21.4 12084 2863 0.060 0.051 0.113 1.112 4.20 −4.22

R1 = ∑||Fo| − |Fc||/∑|Fo|. bwR2 = {∑[w(Fo2 − Fc2)2]/∑w(Fo2)2}1/2. B


Article

Inorganic Chemistry

atomic radius of the lanthanide ion (from La3+ to Eu3+). The partial thermal decomposition of Ln(IO3)3(HIO3)1.33 lead to Ln(IO3)3 phases, which have been indexed as the already characterized αLn(IO3)3,17 isostructural to the γ-La(IO3)3 centrosymmetric phase.20 These compounds crystallize in the monoclinic space group P21/c with the following cell parameters: a = 7.3427(9) Å, b = 8.684(1) Å, c = 13.741(2) Å, β = 99.913(8)°, and V = 863.0(4) Å3 for γ-La(IO3)3 and a = 7.2344(3) Å, b = 8.5640(3) Å, c = 13.5477(5) Å, β = 99.976(2)°, and V = 826.7(1) Å3 for α-Nd(IO3)3. The cell parameters of other lanthanides are reported in Table S1 and Figure S1. For the doped phases β-La1−xNdx(IO3)3 and β-La1−xYbx(IO3)3, the powder patterns (Figures 8 and 9, presented in the Results and Discussion section) were indexed in the monoclinic space group P21 using crystal data of the β-La(IO3)3 phase20 in Winplotr [a = 7.2539(4) Å, b = 8.5360(5) Å, c = 13.5018(7) Å, and β = 97.499(2)°]. All refinements were performed with Fullprof25 by the Le Bail method, according to the neodymium (final x molar fraction by elemental analysis) or ytterbium (nominal x molar fraction) concentrations, and are presented in the Table S2 and Figure S2. Thermal Analyses. Thermal analyses were carried out on a Netzsch DTA-DSC 404S apparatus for differential scanning calorimetry (DSC) analyses. DSC thermal analyses were measured in the 25−500 °C range, in an argon flow, at 5 °C min−1 for the heating and cooling rates. SHG. Kurtz and Perry powder tests were done, leading to a qualitative estimation of the intensity of the SHG signal,26 using the fundamental beam emitted by a Q-switched, mode-locked Nd3+:YAG laser operating at 1.064 μm and generating pulses of 150 ps duration every 200 ms. So, the NLO efficiency was estimated by a visual comparison. For all samples, the same quantity of product was ground and sieved in the same manner in order to make a comparison, placed between two glass sheets, and then put in front of an IR beam (λ = 1.064 μm). In order to determine the optical damage threshold, the energy of the laser emission was gradually increased until the samples became brown. Luminescence Spectroscopy. Fluorescence spectra and decays at room temperature of β-La(IO3)3:Ln3+ were measured in the 4F3/2 and 2F5/2 multiplets of the Nd3+ and Yb3+ excited states, respectively, with a tunable dye laser from Laser Analytical Systems pumped by a frequency-doubled Nd:YAG laser from BM Industries delivering pulses of 10 ns duration with a 10 Hz repetition rate. The visible radiation from the dye laser was approximately 800 nm for Nd3+. For Yb3+, the 696 nm visible laser light was converted to near-IR at approximately 980 nm with a hydrogen-cell Raman shifter. The luminescence was detected by a Hamamatsu R1767 photomultiplier through a HRS2 Jobin-Yvon monochromator equipped with a 1 mm blazed grating. The resulting signal was then processed using a SRS250 gated integrator and a boxcar averager from Stanford Research Systems, providing an integrated signal to a DAC card coupled to a computer for fluorescence spectral measurements. The time evolutions of the fluorescence were recorded with a Lecroy 9410 digital oscilloscope coupled to the same computer.

difference Fourier maps, were considered by poling balance. Furthermore, the carrier O atoms were assigned by considering the longest I−O bond lengths. So, one H atom is carried on the O33 atom [I3−O33 bond length = 1.863(1) Å]. The second one is disordered and carried on the three O11 atoms of the I1 atom located on the 3-fold axis. The three I1− O11 bond lengths are equal to 1.848(1) Å, and the La−O11 bond length is the longest La−O distance. Crystal data for Pr(IO3)3(HIO3), Nd(IO3)3(HIO3), Pr(IO3)3(HIO3)1.33, and Nd(IO3)3(HIO3)1.33 are summarized in Table 1. Powder X-ray Diffraction (XRD) Analyses of Ln(IO3)3(HIO3), Ln(IO 3 ) 3 (HIO 3 ) 1.33 , α-Ln(IO 3 ) 3 , β-La 1−x Nd x (IO 3 ) 3 , and βLa1−xYbx(IO3)3. Powder XRD patterns were collected at ambient conditions using either a Bruker D8 or a Siemens D5000 diffractometer with Cu Kα1 radiation (λ = 1.54056 Å, 40 mA, 40 kV), in the 2θ scan range (10−110°) and with a step size of 0.008 or 0.016°. The powder XRD patterns of Ln(IO3)3(HIO3) (with Ln = Pr, Nd, Sm, and Eu) and Ln(IO3)3(HIO3)1.33 (with Ln = Ce, Pr, Nd, Sm, and Eu) are similar to the patterns of La(IO3)3(HIO3) and La(IO3)3(HIO3)1.33, as shown on Figures 2 and 3, respectively. The

Figure 2. Powder XRD patterns (λCu Kα1 = 1.54056 Å) of La(IO3)3(HIO3) (dark line) and Ln(IO3)3(HIO3) with Ln = Pr (blue), Nd (yellow), Sm (red), and Eu (green). The two insets show enlargements of the patterns at 23−27° and 30−34°, respectively.

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RESULTS AND DISCUSSION Crystal Structures of the Ln(IO3)3(HIO3) and Ln(IO3)3(HIO3)1.33 Phases (with Ln = Pr and Nd). Ln(IO3)3(HIO3) and Ln(IO3)3(HIO3)1.33 crystallize in the centrosymmetric monoclinic space group P21/c and in the noncentrosymmetric trigonal space group R3c, respectively, and are isostructural with the corresponding lanthanum phases.19,20 All phases reveal a three-dimensional network of ionocovalent strong bonds where isolated lanthanum polyhedra are linked by bridging iodate groups. The M−O and M···M distances vary with the lanthanide ions (Table 2). Ln(IO3)3(HIO3)1.33 crystal structures are less compact (Dx = 4.50−4.66 g cm−3) than the Ln(IO3)3(HIO3) centric structure (Dx = 5.09−5.26 g cm−3) in which the distances between Ln atoms connected by iodate bridges are shorter. While the Ln(IO3)3(HIO3)1.33 acentric

Figure 3. Powder XRD patterns (λCu Kα1 = 1.54056 Å) of La(IO3)3(HIO3)1.33 (dark line) and Ln(IO3)3(HIO3)1.33 with Ln = Ce (purple), Pr (blue), Nd (yellow), Sm (red), and Eu (green). The two insets show enlargements of the patterns at 23−26° and 26−30°, respectively. patterns present a slight shift of the peaks to higher-angle values corresponding to a decrease of the lattice parameters and the size of C


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Inorganic Chemistry

acentric β-La(IO3)3 exhibiting an SHG efficiency (green signal). Furthermore, thermal analysis of the so-obtained Pr(IO3)3 phase shows no peak either by heating or by cooling (Figure 5). This is the significant difference compared to

Table 2. Ln−O Bond Lengths and Ln···Ln Distances (Å) in La(IO3)3(HIO3), Pr(IO3)3(HIO3), Nd(IO3)3(HIO3), La(IO3)3(HIO3)1.33, Pr(IO3)3(HIO3)1.33, and Nd(IO3)3(HIO3)1.33 compound

M−O (Å)

M···M (Å)

La(IO3)3(HIO3) Pr(IO3)3(HIO3) Nd(IO3)3(HIO3) La(IO3)3(HIO3)1.33 Pr(IO3)3(HIO3)1.33 Nd(IO3)3(HIO3)1.33

2.428(3)−2.674(3) 2.386(4)−2.652(4) 2.368(5)−2.634(6) 2.490(1)−2.753(1) 2.46(1)−2.78(1) 2.45(1)−2.71(3)

5.739(1)−7.626(1) 5.710(1)−7.583(1) 5.699(1)−7.551(1) 7.058(1)−7.598(1) 7.063(1)−7.515(1) 7.080(2)−7.472(1)

structures are obtained by quick evaporation of the solution (kinetically favored phases), the denser centrosymmetric structures are the thermodynamically stable phases. Thermal Analyses of Ln(IO3)3(HIO3)y (y = 1 or 1.33) and XRD Studies of the Resulting Phases. The studies of Ln(IO3)3(HIO3)y with Ln = Ce, Pr, Nd, Sm, and Eu are based on comprehensive analyses realized by thermogravimetric− differential thermal (TG−DTA) and DSC analyses and in situ temperature-dependent powder XRD experiments for the La(IO3)3(HIO3) and La(IO3)3(HIO3)1.33 compounds, which lead to a better understanding of their similar thermal behaviors and the structural evolution of the different phases.20 Because all Ln(IO3)3(HIO3)y DSC analyses show a similar feature, the results are presented only for the Pr(IO 3 ) 3 (HIO 3 ) 1.33 compound. During heating, the DSC curve (Figure 4) shows

Figure 5. DSC curves of β-La(IO3)3 (dark curve) showing a phase transition to γ-La(IO3)3 and Ln(IO3)3 with Ln = Ce (red curve) and Pr (green curve) without phase transition.

La(IO3)3 (black lines) where a reversible phase transition from β-La(IO3)3 to γ-La(IO3)3 occurs at 185 °C (by heating) and 140 °C (by cooling). So, all Ln(IO3)3 phases obtained from decomposition of Ln(IO3)3(HIO3)y (y = 1 or 1.33) with Ln = Ce, Pr, Nd, Sm, and Eu studied by DSC analyses show the same feature: no transition is observed (Figure 5). These phases have been previously characterized as α-Ln(IO3)3 centric structures.17 Starting from the monometallic Ln(IO3)3(HIO3)y phase, no acentric isostructural phase of β-La(IO3)3 has been obtained. So, we have prepared La1−xLnx(IO3)3(HIO3)y solid solutions with different molar fractions x, in order to follow the thermal behaviors and structural features of La1−xLnx(IO3)3 compounds obtained from the partial thermal decompositions of these solid solutions. Thermal Analyses of La1−xLnx(IO3)3(HIO3)y Solid Solutions (y = 1 or 1.33) and XRD Studies of the Resulting Phases. La1−xLnx(IO3)3(HIO3)y (with Ln = Ce, Pr, Nd, Sm, Eu, Gd, and Yb) have been decomposed by heating in an oven at 480 °C to eliminate 1 or 1.33 molecule of HIO3 respectively, and enable one to obtain the La1−xLnx(IO3)3 phases. DSC analyses of La1−xLnx(IO3)3 solid solutions show a behavior similar to that of β-La(IO3), as shown in Figure 6 (with x = 0.033, 0.057, and 0.108 for the Nd ions). This reversible phenomenon is attributed to the β-La1−xLnx(IO3)3 ⇄ γLa1−xLnx(IO3)3 structural transition. As observed, the transition temperatures vary with the lanthanide concentration in the solid solution, the endothermic (by heating) and exothermic (by cooling) peaks are shifted progressively to lower temperature (to room temperature) as the concentration increases. For the highest molar fractions, the intensity of the DSC peak decreases and disappears. Measurements on a DSC apparatus allowing one to work at lower temperature (from +100 °C to −100 °C with liquid nitrogen) for compounds with high Nd3+ concentration have been performed and confirm the absence of transition. There is a limited substitution fraction in lanthanide ions for which the transition is observed. This limit of substitution decreases with the cationic radius of the lanthanide

Figure 4. DSC curves of Pr(IO3)3(HIO3)1.33 by heating (dark curve) and cooling (blue curve).

two endothermic peaks at 170 °C (maximum at 220 °C) and 310 °C (maximum at 340 °C), corresponding to a departure of 0.33 molecule of HIO3. Then, from 350 °C up to 450 °C (maximum at 425 °C), the intense peak corresponds to the departure of 1 molecule of HIO3. The resulting formulation is Pr(IO3)3. During cooling, no peak is observed. The powder XRD analysis shows that the Pr(IO3)3 phase is not isostructural at room temperature with β-La(IO3)3 but with the hightemperature γ-La(IO3) 3 phase. Accordingly, this phase corresponds to the α-Pr(IO3)3 centric structure previously characterized.17 It crystallizes in the monoclinic space group P21/c with the following cell parameters: a = 7.2616(4) Å, b = 8.5741(5) Å, c = 13.5781(8) Å, β = 100.085(3)°, and V = 832.3(2) Å3. The centrosymmetry is confirmed by the negative SHG activity (Kurtz and Perry powder test),26 compared to the D


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Inorganic Chemistry

Figure 6. DSC curves of β-La(IO3)3 (dark curves) and βLa1−xNdx(IO3)3 with different neodymium molar fractions x [0.033 (red), 0.057 (green), and 0.108 (blue)] versus temperature showing a reversible thermal behavior. The exothermic and endothermic peaks are shifted to lower temperature as the concentration increases.

ion (i.e., as the atomic number of the lanthanide increases) and therefore when the difference with the ionic radius of lanthanum increases (Tables 3 and S3). Figure 7 shows variation of the endothermic (a; by heating) and exothermic (b; by cooling) temperatures for the reversible transition of β-La1−xLnx(IO3)3 in γ-La1−xLnx(IO3)3 (with Ln = Ce, Pr, Nd, Sm, and Yb) according to the lanthanide molar fraction. Some compounds present very broad peaks, so it was easier to collect the temperatures on the top and not at the onset of the peaks. The endothermic and exothermic temperatures decrease linearly from 185 to 97 °C with heating and from 140 to 61 °C with cooling. The slopes of the endothermic (Figure 7a) or exothermic (Figure 7b) peaks depend on the lanthanide inserted in the β-La(IO3)3 matrix. The slope increases when the cationic radius of the lanthanide ion and the limit of substitution decrease, i.e., when the difference with the ionic radius of lanthanum increases. Powder XRD of La1−xNdx(IO3)3 and La1−xYbx(IO3)3. The β-La1−xNdx(IO3)3 (Figure 8) and β-La1−xYbx(IO3)3 (Figure 9) phases were characterized by powder XRD. Figure 8 shows a slight shift of the diffraction peaks to the wide angle up to a Nd3+ molar fraction equal to 0.298 [β-La0.702Nd0.298(IO3)3], which corresponds to a decrease of the lattice parameters. For a higher Nd3+ concentration, we observe the appearance and disappearance of the diffraction peaks, which indicates a structural change. As seen in Figure 10, the β-La1−xNdx(IO3)3 phase follows Vegard’s law up to x = 0.298. At x equal 0.355 and 0.495 for Nd3+, a mixture of β-La1−xNdx(IO3)3 and γLa1−xNdx(IO3)3 solid solutions is observed. The latter

Figure 7. Endothermic (a) and exothermic (b) temperatures for the reversible transition versus lanthanide ion nominal molar fraction or βLa1−xLnx(IO3)3 with Ln = Ce (dark circles), Pr (blue), Nd (red), Sm (green), and Yb (pink). The curves represent the linear fit calculated for each lanthanide ion, and the slopes increase when the cationic radius of the ion decreases. The difference between the curves and experimental points comes from the difference with the nominal and final molar fractions in the lanthanide ions.

corresponds also to α-Nd1−yLay(IO3)3 (with y = 1 − x) issued from pure α-Nd(IO3)3 (monoclinic space group P21/c)17 different from α-La(IO3)3 characterized by Ok and Halasyamani, which crystallized in the monoclinic space group Cc.18,20 Then, from x = 0.72, only the γ-La1−xNdx(IO3)3 solid solution is observed. In Figure 9, β-La1−xYbx(IO3)3 shows the same phenomenon with a solid solution of up to x = 0.05 and then a mixture of β-La1−xYbx(IO3)3 and γ-La1−xYbx(IO3)3 solid solutions for x ranging between 0.09 and 0.15 for Yb3. For x = 0.20, only a γ-La1−xYbx(IO3)3 solid solution is obtained. The substitution limits of the lanthanide ions in the βLa1−xLnx(IO3)3 phase observed in DSC analyses are in accordance with the powder XRD studies. This limit of substitution decreases with the cationic radius of the lanthanide ion. In Table 4, a summary of all anhydrous and nonprotonated lanthanum and lanthanide iodate phases and other isostructural compounds reported in the literature and in this paper is

Table 3. Molar Fraction Dependence of Lanthanide Ions for β-La1−xLnx(IO3)3 on the Reversible Transition lanthanide atomic number Z Ri (with CN = 8) substitution limita

Ce

Pr

Nd

Sm

Eu

Gd

Yb

58 1.143 0.5−0.7

59 1.126 0.3−0.5

60 1.109 0.206−0.298

62 1.079 0.12−0.15

63 1.066 0.09−0.15

64 1.053 0.07−0.1

70 0.985 0.09−0.12

a

Intervals are provided; no intermediate-doped compounds have been prepared. For the lower limit, transition is observed, and for the upper limit, transition is unobserved. E


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Inorganic Chemistry

Figure 8. Powder XRD patterns (λCu Kα1 = 1.54056 Å) of β-La(IO3)3 (bottom dark line), α-Nd(IO3)3 (top dark line), and La1−xNdx(IO3)3 (intermediate colored lines) (final molar fraction x = 0.033, 0.057, 0.108, 0.189, 0.206, 0.298, 0.355, 0.497, and 0.72). The inset shows enlargements of the patterns from 31 to 41°. From x = 0.033 to 0.298, only β-La1−xNdx(IO3)3 is observed; at x = 0.355, a mixture of βLa1−xNdx(IO3)3 and γ-La1−xNdx(IO3)3 solid solutions is observed; then from x = 0.497, only a γ-La1−xNdx(IO3)3 solid solution is observed.

Figure 10. Variation of the V cell volumes of β-La1−xNdx(IO3)3 solid solutions, refined by the Le Bail method in the noncentrosymmetric space group P21 for different final atomic concentrations in Nd3+. This variation follows Vegard’s law (V = 828.6−30.52x) up to x = 0.298.

0.15, and 0.2) upon photoexcitation at 800 nm of the 4F3/2 excited multiplet state of the Nd3+ ion show an intense band centered at approximately 1057 nm with different maxima, as shown in Figure 11 (at 1047, 1057, 1067, and 1080 nm). These maxima originate from the 4F3/2 → 4I11/2 transition. The 4F3/2 and 4I11/2 multiplet states split into two and six levels, respectively, and the number of the clearly observed Stark components in β-La1−xNdx(IO3)3 is 4. The fluorescence decays in β-La1−xNdx(IO3)3 were measured for several Nd3+ concentrations. The decays of all of these emissions are monoexponential and can be fitted by an exponential function: y = Ae−x/τ with τ corresponding to the Nd3+ fluorescence lifetime. All of the decays are summarized in Table 5. A nonradiative energy transfer occurs and increases with the Nd3+ concentration (Figure 13), involving a decrease of the Nd3+ fluorescence decays. A useful Nd3+ molar fraction of 0.02 and less for these hosts is estimated for laser applications. For β-La1−xNdx(IO3)3, the decays are similar to those of the other iodate compounds α-Y(IO3)3, β-Y(IO3)3, AgGd(IO3)4, AgLa(IO3)4, and α-La1−xNdx(IO3)3 already studied.15,16,20 Luminescence Spectroscopy of β-La1−xYbx(IO3)3. The room temperature emission spectra of β-La1−xYbx(IO3)3 (nominal molar fraction x = 0.01, 0.02, 0.03, 0.04, and 0.05) upon photoexcitation at 953 nm of the 2F5/2 excited multiplet state of the Yb3+ ion show a broad band between 960 and 1040 nm with a maximum characterized by a narrow emission line at approximately 975 nm due to the 2F5/2 → 2F7/2 transition, as shown in Figure 12. The 975 nm peak wavelength (“zero-line”) belongs to both emission and absorption, so it is hardly usable for laser emission. The assignments of the bands are not possible at room temperature, and only measurements at low temperature allow it. The Yb ion has several advantages compared to the Nd ion because of its very simple energy scheme constituted with only one excited state and the ground state ( 2F 5/2 → 2F7/2 transition): no excited-state absorption and no concentration quenching within a large domain of concentration. Furthermore, the small quantum defect contributes to weak thermal effects, so that Yb3+-doped materials are interesting for efficient high-power continuous-wave lasers. The Yb3+-doped materials have another advantage; they bring new advances in diode-

Figure 9. Powder XRD patterns (λCu Kα1 = 1.54056 Å) of β-La(IO3)3 (bottom dark curve) and β-La1−xYbx(IO3)3 (colored and top curves) (nominal molar fraction x = 0.02, 0.05, 0.09, 0.15, and 0.20). The inset shows enlargements of the patterns from 31 to 41°. Between x = 0.09 and 0.15, a mixture of β-La1−xLnx(IO3)3 and γ-La1−xLnx(IO3)3 solid solutions is observed; then from x = 0.20, only a γ-La1−xYbx(IO3)3 solid solution is observed.

presented. Only lanthanum compounds present polymorphic acentric phases, in contrast to other lanthanide structures, which are all centrosymmetric. As suggested by the evolution of the transition temperature and the limit of substitution of lanthanide in the β-La1−xLnx(IO3)3 phases, the noncentrosymmetry is controlled by the size of La and Ln cations. This phenomenon was already demonstrated in the A2Ti(IO3)6 (A = Li, Na, K, Rb, Cs, and Tl) system, with the macroscopic polarity being controlled by the A+ cation.27 The materials with the smaller cations, Li+ and Na+, are noncentrosymmetric and polar, whereas the compounds with the larger cations, K+, Rb+, Cs+, and Tl+, are centrosymmetric and nonpolar. Recently, the same feature has been obtained in the A2Pt(IO3)6 (with A = H3O, Na, K, Rb, and Cs) isostructural compounds.28 Luminescence Spectroscopy of β-La1−xNdx(IO3)3. The room temperature emission spectra of β-La1−xNdx(IO3)3 (nominal molar fraction x = 0.01, 0.02, 0.03, 0.04, 0.05, 0.09, F


Article

Inorganic Chemistry Table 4. Summary of All Anhydrous Lanthanum and Lanthanide Iodate Phases and Isostructural Compounds

a

Cc

P21

P21/c

P21/n

α-La(IO3)3a,b α-La1−xLnx(IO3)3b

β-La(IO3)3b β-La1−xLnx(IO3)3c

γ-La(IO3)3b γ-La1−xLnx(IO3)3 = α-Ln1−yLay(IO3)3c α-Y(IO3)3d,e,f α-Ln(IO3)3d,e,f α-Y1−xLnx(IO3)3g

β-Ln(IO3)3e,f β-Y(IO3)3e,f β-Y1−xLnx(IO3)3g Bi(IO3)3h

Reference 18. bReference 20. cThis work. dReference 29. eReference 30. fReference 17. gReference 16. hReference 31.

pumped ultrashort sources in the femtosecond scale of time (due to a wide fluorescence spectrum and the superposition of several emissions with close energy). As for Nd3+, the excited and ground states of Yb3+ split into three and four Stark levels. As for Nd3+, the fluorescence decays τ in β-La1−xYbx(IO3)3 were measured for several Yb3+ concentrations. The values of τ are displayed in Table 5. The fluorescence decays increase continuously from 0.01 to 0.04 with a maximum of 688 μs for 0.04 and then decrease at higher concentration. The decrease of the lifetime is due to energy migration and trapping effects (Figure 13). The 2F7/2 → 2F5/2 radiative reabsorption can be in

Figure 11. Emission spectra of β-La1−xNdx(IO3)3 under an 800 nm excitation wavelength at room temperature.

Table 5. Room Temperature Dependence of the 4F3/2 Fluorescence Decay Time (τ) versus the Molar Fraction x of Nd3+ in β-La1−xNdx(IO3)3 upon Photoexcitation at 800 nm and Room Temperature Dependence of the 2F5/2 Fluorescence Decay Time (τ) versus Molar Fraction x of Yb3+ in β-La1−xYbx(IO3)3 upon Irradiation at 953 nm molar fraction x in βLa1−xNdx(IO3)3

τ (μs)

0.01 0.02 0.03 0.04 0.05 0.09 0.15 0.20

178 168 181 156 146 142 108 85

molar fraction x in βLa1−xYbx(IO3)3

τ (μs)

0.01 0.02 0.03 0.04 0.05

473 549 634 688 641

Figure 13. Lifetimes of β-La1−xNdx(IO3)3 (dark circles) and βLa1−xYbx(IO3)3 (blue triangles) with different molar fractions x in Nd3+ and Yb3+ at room temperature.

competition with energy migration and trapping effects, which increase the apparent measured decay in bulk samples32 and very often lead to a maximum lifetime such as in the iodates αY(IO3)3, β-Y(IO3)3, and α-La1−xYbx(IO3)3.16,20 For the development of laser applications, the ideal concentration of Yb3+ is difficult to optimize (due to the quasi-three-level laser). The β-La(IO3)3 host doped with Ln3+ shows long Ln3+···Ln3+ intermetallic distances [5.872(7) Å] compared to the αLa(IO3)3 host doped with Nd3+ and Yb3+, where the shorter Ln3+···Ln3+ intermetallic distances are equal to 4.559(1) Å.

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CONCLUSION These last years have permitted the discovery of no less than six lanthanum iodates, La(IO3)3(HIO3), La(IO3)3(HIO3)1.33, and α-, β-, γ-, and δ-La(IO3)3, as well as new families of lanthanide iodates, Ln(IO3)3(HIO3), Ln(IO3)3(HIO3)1.33, and doped lanthanum iodates La1−xLnx(IO3)3(HIO3)1.33 and β- and γLa1−xLnx(IO3)3. All of these phases were characterized by single-crystal or powder XRD, and all of them present a threedimensional network of ionocovalent strong bonds. Furthermore, the La(IO3)3(HIO3)1.33, α- and β-La(IO3)3, and β-

Figure 12. Emission spectra of β-La1−xYbx(IO3)3 under a 953 nm excitation wavelength at room temperature. G


Article

Inorganic Chemistry La1−xLnx(IO3)3 phases are noncentrosymmetric. The iodate hosts present remarkable transparency from visible light to the beginning of the far-IR (12 μm), opening potential applications for telecommunications and spectroscopy. As previously reported,20 a comparison of the intensities of SHG light generated leads to the following result: β-La(IO3)3 < La(IO3)3(HIO3)1.33 < α-La(IO3)3 ≈ α-LiIO3. This comparison allows us to give an estimation of the maximal nonlinear coefficients dij of several picometers per volt for these iodates as determined for α-LiIO3 on a single crystal (d31 = 7.11 pm V−1 and d33 = 7.02 pm V−1 for α-LiIO3)33 and estimated for αLa(IO3)3 [(deff)exp = 23 pm V−1].18 Consequently, only crystal with millimetric size will enable measurement of the phasematching properties of sum- and difference-frequency generations, as well as dispersion equations of the refractive indices and of the nonlinear coefficients of these compounds over their transparency range.34−36 Thermal analyses and powder XRD experiments of La(IO3)3(HIO3)y and La1−xLnx(IO3)3(HIO3)y (y = 1 and 1.33) show a HIO3 loss, leading to the formation of γ-La(IO3)3/γLa1−xLnx(IO3)3, followed by a phase transition in β-La(IO3)3/ β-La1−xLnx(IO3)3, stable at room temperature. For pure phases, a reversible phase transition from γ-La(IO3)3 to β-La(IO3)3 occurs at 140 and 185 °C during cooling and heating, respectively. For the doping compounds, the reversible transition of β-La1−xLnx(IO3)3 in γ-La1−xLnx(IO3)3 (with Ln = Ce, Pr, Nd, Sm, Eu, Gd, and Yb) depends on the lanthanide molar fraction; the endothermic and exothermic temperatures decrease linearly from 185 to 97 °C with heating and from 140 to 61 °C with cooling. There is a limited substitution fraction in lanthanide ions for which the transition is observed. This limit of substitution decreases with the cationic radius of the lanthanide ion and therefore when the difference with the ionic radius of lanthanum increases. On the other hand, thermal treatment of Ln(IO3)3(HIO3)y (y = 1 and 1.33) leads to stable phases without any reversible transition of α-Ln(IO3)3. Only lanthanum iodate presents polymorphic acentric phases, while the other monometallic rare-earth structures are all centrosymmetric. As suggested by the evolution of the transition temperature and the limit of substitution of lanthanide in the β-La1−xLnx(IO3)3 phases, the noncentrosymmetry of the structures is controlled by the size of the lanthanum and lanthanide cations. This similar phenomenon was already demonstrated in A2Ti(IO3)6 (A = Li, Na, K, Rb, Cs, and Tl), with the macroscopic polarity being controlled by the A+ cation. The materials with smaller cations, Li+ and Na+, are noncentrosymmetric and polar. The photophysical properties of β-La1−xNdx(IO3)3 and βLa1−xYbx(IO3)3 were studied for several Nd3+/Yb3+ concentrations. The luminescence of doped Nd3+ and Yb3+ ions in βLa(IO3)3 shows fluorescence decay times of 85−178 μs for Nd3+ and 473−688 μs for Yb3+ similar to other iodate compounds such as α-La(IO3)3, α-Y(IO3)3, β-Y(IO3)3, AgGd(IO3)4, and AgLa(IO3)4. The metal iodates reveal a wide domain of transparency for considering NLO and laser applications interesting as potential self-frequency doubling materials.

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Powder X-ray refinement and graphical variation of the cell parameters for α-Ln(IO3)3 and β-La1−xLnx(IO3)3 and molar fraction dependence of the lanthanide ions for βLa1−xLnx(IO3)3 on the reversible transition (PDF) X-ray crystallographic data in CIF format (CIF) X-ray crystallographic data in CIF format (CIF) X-ray crystallographic data in CIF format (CIF) X-ray crystallographic data in CIF format (CIF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +33 4 76 88 78 04. Fax: +33 4 76 88 10 38. Notes

The authors declare no competing financial interest.

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Differences and Similarities between Lanthanum and Rare-Earth

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