Lithium silicate, LiAlSi₄O₁₀ (petalite)—a novel monoclinic SRS-active crystal

In the history of physics the study of natural crystals was the beginning of many of its modern directions. To a certain extent this is valid for crystals of optical silicates. The birth of spontaneous Raman scattering in solids was associated with α–quartz (SiO₂) in 1928 [1, 2]. The start of nonlinear optics is also connected with natural α–quartz, when its χ⁽²⁾–nonlinearity of the second harmonic generation (SHG) of the ruby laser (Cr³⁺:Al₂O₃) radiation was discovered in 1961 [3]. The χ⁽³⁾–nonlinearity was manifested in the discoveries of recent years in the phenomenon of the stimulated Raman scattering (SRS) in natural silicates like α–quartz [4], topaz, Al₂(F,OH)₂SiO₄ [5], phenakite, Be₂SiO₄ [6], and zircon, ZrSiO₄ [7]. Up to now SRS–active synthetic silicate crystals e.g. Ca₂ZnSi₂O₇ [8], Ca₂Ga₂SiO₇ [8], Bi₄Si₃O₁₂ [9], and Bi₁₂SiO₂₀ [10, 11] are also known, where the latter three are also laser-active doped with Nd³⁺ ions (see, e.g [10, 12, 13]). It is appropriate to mention here the particular role of crystalline silicates in the development and establishment of modern laser physics and nonlinear optics. This contribution is illustrated in table 1, which shows the currently known laser silicates with their generating ions of the trivalent lanthanides (Ln³⁺) and chromium. This work continues our search study for SRS-active natural silicates.

Since the first investigation with the aid of x-ray diffraction in 1930 [19] petalite, LiAlSi₄O₁₀, which is a commercially important source of lithium, has been subject of various studies focusing mainly on crystallographic and crystal chemical aspects [20–23]. This mineral crystallizes in the monoclinic space group P2/a with lattice parameters a₁ = 11.754 Å, a₂ = 5.139 Å, a₃ = 7.630 Å, α₂ = 113.04° at room temperature [22]. However, petalite exhibits a pronounced orthorhombic pseudo-symmetry, which is broken by the fully ordered distribution of Li⁺ and Al³⁺ cations [23]. The crystal structure can be classified as a 3D framework of corner-connected TO₄ tetrahedra, where T = Li, Al or Si (figure 1). Alternatively, as there is perfect ordering of the cations on distinct crystallographic sites, petalite can be considered as a layer silicate. Following this interpretation, the structure is built up from folded [Si₄O₁₀] layers parallel to (0 0 1) which are linked by LiO₄– and AlO₄–tetrahedra. In a study to establish an internally consistent thermodynamic database for lithium-containing minerals the bulk modulus was measured in a multi-anvil cell and thermal expansion coefficients were determined using high temperature x-ray diffraction [24], which they combined with an earlier calorimetric study [25]. Poolton et al [26] carried out extensive studies on the IR stimulated luminescence behavior of this mineral. A sequently detailed study of the thermo–luminescence, the optical absorption, and electron paramagnetic resonance was performed by [27]. And most recently distinct crystal-physical properties were investigated like the elastic stiffness coefficients and thermal expansion [18]. In table 2 the physical properties at room temperature of LiAlSi₄O₁₀ are summarized.

Table 2. Selected known physical properties of petalite, LiAlSi₄O₁₀.

Characteristics
Space group [19, 22]	$C_{2h}^{4}-P2/a$ (No. 13)
Unit cell parameters cell, Å [18]	a1 = 11.724(2); a2 = 5.128(1); a3 = 7.622(2);
	α2 = 113.07(2)°a
Formula units per unit cell [22]b	Z = 2
Density, g cm−3 [18]	ρx = 2.392
Melting point, °C	≈1400c
Hardness (Mohs scale)	6–6.5c
Elastic constants (average), GPa [18]	c11 = 91.5(2); c22 = 93.04(4); c33 = 152.5(9);
	c44 = 29.37(6); c55 = 69.7(2); c66 = 52.7(1);
	c12 = 0.1(1); c13 = 69.0(4); c23 = 6.5(2);
	c15 =  −0.7(2); c25 =  −1.5(4); c35 =  −0.7(2);
	c46 = 2.4(1)
Linear thermal expansion, 10−6 K−1 [18]	α11 = 0.974(1); α22 = 11.689(1);
	α33 =  −7.351(1); α13 = 0.244(1);
UV-border of the optical transmission spectrum, μm	≈0.2
Linear optical character	Biaxial positive
Refractive indicesc	nα = 1.504; nβ = 1.510: nγ = 1.516;
	2 V = 82°–84°
Nonlinearity	χ(3)
Energy of SRS-promoting vibration modes, cm−1 [this work]	ωSRS1  ≈  490; ωSRS2  ≈  357
Phonon spectra extension, cm−1d	≈  1150

Atom	X	y	z	CN	SS	WN
Li	0.25	0.2553	0	4	C2	2e
Al	0.25	0.7564	0	4	C2	2e
Si1	0.9980	0.5128	0.2896	4	C1	4g
Si2	0.1477	0.0099	0.2896	4	C1	4g
O1	0	0.5	0.5		Ci	2b
O2	0.25	0.9654	0.5		C2	2f
O3	0.0938	0.3012	0.2704		C1	4g
O4	0.3617	0.5358	0.1342		C1	4g
O5	0.0381	0.8011	0.2518		C1	4g
O6	0.2076	0.9779	0.1353		C1	4g

^aAccording to [22]: a₁ = 11.754 Å; a₂ = 5.139 Å; a₃ = 7.630 Å; α₂ = 113.04°. ^bFractional position parameters, oxygen coordination number (CN), site symmetry (SS), Wyckoff notation (WN) [23]: ^cMineral Data Publishing (2001). ^dRRUFF Sample Data-Integrated Database of Raman Spectra, XRD and Chemistry Data for Minerals, Petalite RO40100.

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Colourless single crystals of natural petalite with gem quality were chosen for our SRS spectroscopy from a locality in Laghman, Afganistan. Its lattice parameters a₁ = 11.724(2) Å, a₂ = 5.128(1) Å, a₃ = 7.622(2) Å, α₂ = 113.07(2) obtained from x-ray diffraction experiments on a four-circle diffractometer equipped with graphite-monochromatized MoKα–radiation agree well with the literature values [21, 22] (see also table 2). Electron microprobe analyses revealed a nearly ideal composition with traces of Na, K, Mn, Mg, Ca, Fe, Ni, and Cr (in total less than 0.05 wt.%). The SRS properties reported in this work are referred to a Cartesian reference system (crystal-physical reference system) with basis vectors e_i which are connected to the crystallographic basis vectors a_i according to e₁ || a₁, e₂ || a₂, and e₃ = (e₁  ×  e₂) || $\boldsymbol{a}_{3}^{\ \ *},$ where $\boldsymbol{a}_{3}^{\ \ *}$ denotes the third basis vector of the reciprocal system. This non-standard setting was chosen to take advantage of the perfect cleavage plane parallel to (0 0 1)_x-ray and of the pseudo-orthorhombic symmetry. For the SRS measurements a sample of LiAlSi₄O₁₀ with surface normals along e₁, e₂, e₃ (faces (1 0 0)_phys, (0 1 0)_x-ray and (0 0 1)_x-ray) respectively, and dimensions of 8.91   ×   13.54   ×   6.78 mm³ was prepared by cutting from a large petalite single crystal using a low-speed diamond wire saw and ground on a diamond disc (mesh 1200). All the sample faces were polished using 0.06 μm Al₂O₃ slurry but without an anti-reflection coating.

The investigations of Raman induced χ⁽³⁾–nonlinear interactions in monoclinic LiAlSi₄O₁₀ were carried out at room temperature in a cavity-free excitation scheme using an experimental setup based on a home-made picosecond Nd³⁺:Y₃Al₅O₁₂ laser with an external KTiOPO₄ doubler as the pumping source for λ_p = 0.53207 μm wavelength (see e.g [4, 28]). Two selected Raman-induced χ⁽³⁾–lasing spectra of petalite recorded at two different excitation geometries using a grating monochromater (McPherson Model 270) equipped with a Si–CCD line sensor (Hamamatsu S3924–1024Q) are shown in figures 2 and 3. They clearly demonstrate its two-phonon (ω_SRS1  ≈  490 cm⁻¹ and ω_SRS3  ≈  357 cm⁻¹) high-order Stokes and anti-Stokes generation in the UV and visible range. The results of their analysis are summarized in table 3. Due to the short sample length along e₂ of the investigated natural LiAlSi₄O₁₀ sample, it was difficult to experimentally evaluate its steady-state Raman gain coefficient as it was usually possible in the comparative measurements of optically committed other inorganic SRS-crystals (see, e.g [5–8]). However, on the basis of the χ⁽³⁾–lasing behavior of the petalite and other silicates with the SRS-active tetrahedral SiO₄ units (there are currently known about ten such silicates, among them are synthetic and natural crystals, see table 1 and table 4 [8]) it is safe to assert very roughly that the average gain coefficient of petalite is not below 0.2 cm · GW⁻¹ in the visible spectral range.

Table 4. Known SRS-active natural crystals (minerals) and their nonlinear laser properties.

Natural crystals (minerals)a	Space group	SRS-active vibration modes,cm−1 b	Observed manifestations of χ(2)– and χ(3)–nonlinear laser interactions
LiAlSi4O10 (petalite)	$C_{2h}^{4}-P2/a$	≈490,≈357 (present work)	SRS (present work)
Be2SiO4 (phenakite)	$C_{3i}^{2}-R\bar{3}$	≈876 [6]	SRS, χ(3)–combc [6]
C (diamond)	$O_{h}^{7}-Fd\bar{3}m$	≈1332 [32] d	SRS [32]
Al2(F,OH)2SiO4 (topaz)	$D_{2h}^{16}-Pbnm$	≈265 (≈268),	SRS, THG–SFG f,
		≈238 (≈239),	self–SFG(SRS)g,
		≈503e (≈507e),	χ(3)–comb, χ(3)–cr–casch,
		≈27e (≈29e) [5]	com–phone [5]
SiO2 (α-quartz)	$D_{3}^{4}-P{{3}_{1}}21$i	465.5 [4]	SRS, SHG, THG,
			self–SFG(SRS),
			χ(3)–comb [4, 33]
α–S8 (α-sulphur)	$D_{2h}^{24}-Fddd$	≈468, ≈216 [32]	SRS [32]
α–KAl(SO4)2·12H2O (potassium alum)	$T_{h}^{6}-Pa\bar{3}$	≈990 [34]	SRS, χ(3)–comb [34]
CaCO3 (calcite)	$D_{3d}^{6}-R\bar{3}c$	1086.5 [35], ≈282 [36]	SRS, SHG j, THG k
			FiHG l, self–SFG(SRS),
			χ(3)–comb, χ(3)–cr–casc [32, 35–41]
CaCO3 (aragonite)	$D_{2h}^{16}-Pmcn$m	≈1087 (≈1096),	SRS, THG–SFG,
		≈152 (≈157),	self–SFG(SRS),
		≈205 [42]	χ(3)–comb, χ(3)–cr–casc [42]
SrSO4 (celestine)	$D_{2h}^{16}-Pnma$	≈999 [43]	SRS, THG–SFG,
			self–SFG(SRS),
			χ(3)–comb [43]
ZrSiO4 (zircon)	$D_{4h}^{19}-I{{4}_{1}}/amd$	≈1008 [7]	SRS, χ(3)–comb,
			χ(3)–cr–casc [7]
BaSO4 (barite)	$D_{2h}^{16}-Pnma$	≈999 [44]	SRS, THG–SFG,
			self–SFG(SRS),
			χ(3)–comb [44]
PbCO3 (cerussite)	$D_{2h}^{16}-Pmcn$	≈1054 [45]	SRS, χ(3)–comb [45]
Pb2CO3Cl2 (phosgenite)	$D_{4h}^{5}-P4/mbm$	≈1062, ≈86 [46]	SRS [46]
PbSO4 (anglesite)	$D_{2h}^{16}-Pnma$	≈985 [44]	SRS, χ(3)–comb [44]

^aCrystals are listed by the 'alphabet' of the periodical table of elements. ^bIn parentheses SRS-promoting vibration modes measured at  ≈9 K are given. ^cχ⁽³⁾–comb: spectrum of Stokes and anti-Stokes laser frequency components that span at least one octave (i.e. the highest frequency (energy) component must be at least double the lowest frequency component). ^dMost probably natural crystals of diamond were used in this study [32]. ^eCom-phon: combined SRS-active phonon vibration arising as an interaction of two original SRS-promoting modes of crystal. ^fTHG-SFG: third harmonic generation arising via parametric four-wave mixing (generation) under collinear coherent pumping at two fundamental wavelengths λ_f1 = 1.06415 μm and λ_f2 = 0.53207 μm (externally generated second harmonic generation of the fundamental wavelength λ_f1) of a picosecond Nd³⁺:Y₃Al₅O₁₂ laser [5]. ^gself-SFG(SRS): sum-frequency generation arising from SRS-lasing components and pumping radiation. ^hχ⁽³⁾–cr–casc: cascaded of one or many-step χ⁽³⁾–lasing with the involvement of the interaction of different SRS-promoting vibration modes of the crystal in the photon–phonon generation of high-order Stokes and anti-Stokes components. ⁱOr $D_{3}^{6}-P{{3}_{2}}21.$ ^jSHG: second harmonic generation. ^kTHG: third harmonic generation. ^lFiHG: fifth harmonic generation. ^mFor aragonite a non-standard setting bca, corresponding to the space group symbol Pmcn, is frequently used in the literature.

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Now we will discuss the vibronic nature of the observed SRS-promoting modes of the natural crystal of LiAlSi₄O₁₀. Its primitive unit cell contains two Li⁺, two Al³⁺, eight Si⁴⁺ and twenty O²⁻ ions (see table 2). Lithium and aluminium ions occupy the 2e position of the C₂ symmetry and silicon ions occupy the 4 g positions of the general C₁ symmetry. In the primitive cell there are three different oxygen atoms that differ in their symmetry. Two oxygen atoms occupy the 2b sites of the C_i symmetry, two other oxygen atoms lie on the 2f sites of the C₂ symmetry, and eight oxygen atoms occupy the 4 g sites of the general symmetry 1(C₁). The structure of the petalite originates from the orthorhombic pseudo-symmetry, which is broken by the fully ordered distribution of Li⁺ and Al³⁺ cations [23]. The 3D framework of the petalite structure is built up from the LiO₄ and AlO₄ tetrahedrons which link the SiO₄ tetrahedra constituting the Si₄O₁₀ layer. Such crystal structure of this mineral strongly influences its physical properties, which was considered in [18]. The basis for the calculations of phonons in the unit cell of LiAlSi₄O₁₀ is the above-described structure (see also section 2). The 32 atoms in the primitive cell of petalite (Z = 2) give rise to 3NZ = 96 zone-center degrees of freedom described by the irreducible representation: Γ₉₆ = 21A_g + 24B_g + 24A_u + 27B_u. Three of them, Γ_T = A_u + 2B_u, describe the acoustic phonons and remaining, Γ_O = 21A_g + 24B_g + 23A_u + 25B_u, correspond to the optical modes. The optical modes can be further subdivided among the vibrations of components of the primitive cell:

• Translations of the Li⁺ ions: Γ_T' (Li⁺) = A_g + 2B_g + A_u + 2B_u,
• Translations of the Al³⁺ ions: Γ_T' (Al³⁺) = A_g + 2B_g + A_u + 2B_u,
• Translations of the Si⁴⁺ ions: Γ_T' (Si⁴⁺) = 6A_g + 6B_g + 6A_u + 6B_u,
• Vibrations of the O(1) atoms: Γ_v (O1) = 3A_u + 3B_u
• Vibrations of the O(2) atoms: Γ_v (O2) = A_g + 2B_g + A_u + 2B_u
• Vibrations of the O(3–6) atoms: Γ_v (O3 ÷ O6) = 12A_g + 12B_g + 12A_u + 12B_u.

It should be noted that in the Si₄O₁₀ layer some of the oxygen atoms form the terminal Si–O bonds, other atoms participate in Si–O–Si bridges, and the next atoms form the Si–O–Al and Si–O–Li bridges. Therefore, 78 internal vibrations of the 2  ×  Si₄O₁₀ units are described by the representations: 19A_g + 20B_g + 20A_u + 19B_u. The internal vibrations of the layer are strongly coupled with the translations and librations of the SiO₄–tetrahedra, therefore all the observed IR and Raman bands have a mixed character. The A_g and B_g modes are Raman-active and those of A_u and B_u are IR-active. The first-order spontaneous Raman scattering spectrum of LiAlSi₄O₁₀ is shown in figure 4. The wave numbers of the bands observed in this spectrum will be used in the analysis of the observed SRS components that are related to phonons with ω_SRS1  ≈  490 cm⁻¹ and ω_SRS2  ≈  357 cm⁻¹. This spectrum is typical for the polymeric silicates containing the internal and external phonons originating from the vibrations of the tetrahedral SiO₄ units joined through Si–O–Si oxygen bridges. The following assignment of the bands of a given spectrum to the respective normal vibrations could be proposed [30, 31]:

• The bands at 1138 cm⁻¹ and 1060 cm⁻¹ correspond to the symmetric and asymmetric stretching vibrations ν_s(SiO₄) and ν_as(SiO₄) coupled with the ν_s(Si–O–Si) vibration,
• The band at 790 cm⁻¹ corresponds to the stretching ν(Si–O–Si) mode,
• The strong band at 490 cm⁻¹ originates from the symmetric bending δ_s(SiO₄) vibrations of the silicate tetrahedrons. The satellite of this band at 467 cm⁻¹ corresponds to the B_g–mode of this vibration,
• Raman lines observed in the range between 300–400 cm⁻¹ correspond to the bending vibrations of the SiO₄ and AlO₄ tetrahedra. Therefore the very strong intensity band at 357 cm⁻¹ was described by the δ(SiO₄/AlO₄) vibrations of the B_ig–symmetry,
• Other lines of the lattice modes should be assigned to the lattice modes corresponding to the translations of the Li⁺ and Al³⁺ ions.

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Based on the foregoing, we conclude that the observed SRS-promoting phonon modes ω_SRS1  ≈  490 cm⁻¹ and ω_SRS2  ≈  357 cm⁻¹ can be attributed to the symmetric A_g–bending δ_s(SiO₄) vibration and asymmetric B_g–bending δ_as(SiO₄/AlO₄) vibration, respectively.

We have discovered and investigated the χ⁽³⁾–nonlinear optical potential of monoclinic natural petalite, LiAlSi₄O₁₀, as a novel material for Raman laser converters in the visible spectral range. All recorded Stokes and anti-Stokes lines of picoseconds χ⁽³⁾–lasing were identified and attributed to two SRS-promoting modes (ω_SRS1  ≈  490 cm⁻¹ to the symmetric A_g–vibration and ω_SRS1  ≈  357 cm⁻¹ to the asymmetric B_ig–vibration) of the tetrahedron SiO₄ units of the studied silicate. It would be highly desirable from a practical point of view, if our investigation spurs the growth of large single crystals with the high optical quality of LiAlSi₄O₁₀. Petalite can enrich modern laser physics and nonlinear optics and is a valuable material for their applications. The studies also confirm that obtaining detailed knowledge of the physical properties of natural optical crystals is a persuasive tool in the development of new optical materials for modern laser physics and nonlinear optics and their applications. This is confirmed by the data in table 4, which shows the χ⁽²⁾– and χ⁽³⁾–nonlinear laser properties of the known natural SRS-active crystals. Here it is appropriate to add that some of their synthetic analogues have already been used. Given the heuristic value of such investigations, we will continue our nonlinear-laser search experiments with other natural optical crystals.

This study was supported by research programs of the Institute of Crystallography of the Russian Academy of Sciences, by the Deutsche Forschungsgemeinschaft (Project HA-5137/3), by the Institute of Optics and Atomic Physics of the Technical University of Berlin, by the Institute of Low Temperature and Structure Research of the Polish Academy of Sciences, by the Institute for Laser Science of the University of Electro-Communications in Tokyo, and by the Photon Frontier Network Program of the Ministry of Education, Culture, Sport, Science, and Technology, Japan.