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 Lanthanum gallium silicate ( La2Ga5SiO14 ), or langasite / LGS is a phenomenal invention. Though langasite was initially developed as a laser crystal, it revealed a remarkable combination of piezoelectric qualities, which opened up new opportunities to use the LGS crystal in radio frequency devices. The main applications of langasite crystals now is SAW filters and SAW resonators. Experts agree that LGS is the best choice for high-efficiency broad band frequency devices because it possesses exceptional frequency-temperature characteristics, a very high index of electromechanical coupling and un unparalleled Q-factor.
Lanthanum gallium tantalite (La3Ga5.5Ta0.5O14), or langatate (LGT) is gaining popularity with manufacturers of control and measurement devices. Pressure and vibration sensors produced on the basis of langatate show excellent performance at temperatures up to 600°C, and experts say that a langatate sensor operating at up to 900°C is likely to emerge in the near future. |
| Crystal Structure | trigonal system, group 33
a= 8.1783 c = 5.1014 | | | Growth Method | Czochralski | | | Hardness | 6.6 Moh's | | | Density | 5.754 g/cm3 | | | Melt Point | 1470 o C ( phase transition point: N/A) | | | Thermal expansion (x10-6/ oC) | α 11: 5.10 α 33: 3.61 | | | Acoustic velocity, SAW | 2400 ( m/sec ) | | | Frequency constant, BAW | 1380 ( kHz/mm ) | | | Piezoelectric coupling | K2 (%) BAW: 2.21 SAW: 0.3 | | | Inclusions | N0 | |
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Comparison of Piezoelectric Crystals Used for SAW Applications |
| | Langasite, La2Ga5SiO14 | Quartz, SiO2 | Lithium tetraborate, Li2B2O3 | Lithium tantalate, LiTaO3 | | Index of el.mech. coupling Kemc, % | 15.8 | 7.0 | 24.0 | 47.0 | | Frequency spacing Δf, % | 0.90 | 0.25 | 4.00 | 7.00 | | Q-factor | Q, x103 | 50 | 100 | 10 | | Temp. frequency coef., TFC, x10-1 °C | 1.6 | 0.5 | 6.0 | 4.0 |
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Comparison of Surface Waves in Quartz and Langasite |
| Characteristics | Quartz, SiO2 | LGS,La2Ga5SiO14 | | | Vef | 0°, 132.75°, 0° | 0°, 140°, 22.5° | 0°, 140°, 25° | | K2emc | 0,14 | 0,32 | 0,38 | | SAW velocity | 3157 | 2742 | 2736 | | a2 x 108/°C2 | -3.2 | -7.8 | -6.8 | | TT,°C (temp. coef.) | 25 | 25 | 23 | | E (dielectric const) | 4,92 | - | 27 | | M, degr. (power flow angle) | 0 | 52 | 51 |
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Langatate Comparison of Piezoelectric Crystals Used for Pressure and
Vibration Sensors |
| | Langatate La3Ga5.5Ta0.5O14 | Quartz, SiO2 | APC-850 N-2 | APC-856 | | Relative dielectric constant K=εT 33/ε0, 1 | 80.3 | 4.6 | 1750 | 4100 | | Curie point Tc, °C | - | 570 | 360 | 150 | | Piezoelectric charge constant, 10-12 C/N or m/Vz | _ | _ | _ | | | -d11 | 6.5 | 2.3 | d33=400 | d33=620 | | d14 | 4.7 | 0.9 | d15=590 | d15=710 | | Piezoelectric voltage constant, 10-3 Vm/N or m2/C | _ | _ | _ | | | -g11 | 38 | 58 | g33=26 | g33=18.5 | | g14 | 27.7 | 18 | g15=36 | g15=25 | | Young modulus, 1010 N/m2 | _ | _ | _ | | | YE11 | 11 | 7.8 | 6.3 | 5.8 | | YE33 | 19 | 10.4 | 5.4 | 4.5 | | Frequency constants, Hz-m or m/s | _ | _ | _ | | | Longtitudal mode NL | 2870 | - | 1500 | - | | Thickness mode NT | 1320 | 1666 | 2032 | 1980 | | Elastic compliance, 10-12 m2/N | _ | _ | _ | | | SE11 | 9.0 | 12.8 | 15.3 | 15.0 | | SE33 | 5.2 | 9.6 | 17.3 | 17.0 | | Density ρ, g/cm3 | 6.13 | 2.65 | 7.7 | 7.5 | | Q-value Qm, 1 | 50000 | 106 | 80 | 72 |
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| Langasite as-grown Boules |
| Diameter/Width | 80-100 mm | | Thickness/Length | Up to 150 mm | | Orientation | along <00.1>, <01.1> and <11.0> axis | | Surface quality | As-grown |
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Langasite Substrates for SAW Applications |
| Diameter | 76.2-101.4 mm | | Thickness | 0.5 mm | | Orientation | Primary flat is perpendicular to X' axis with accuracy ±15 min, Yxlt/48.5°/26.6° | | LTV | 2.2 microns for the base 20 mm X 20mm | | Surface quality | No dimples, cracks, unpolished areas, contaminants in working area under x50 magnification. Scratches visible at x50 magnification are allowed in working area if their quantity is lower than 3 on one wafer and less than 20 in a lot of one hundred wafers. | | Edge chipping | No chipping inside the working area and on the primary flat. Chipping can be accepted outside the working area if the width is less than 0.5 mm, and the cumulative length is less than 5 mm. | | Working face | Working area: wafer diameter minus 3 mm |
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Langatate blocks and plates for sensing parts |
| Diameter/Width | Up to 80 mm | | Thickness/Length, X-cut | 5-30 mm | | Thickness/Length, Y-cut | 5-30 mm | | Thickness/Length, Z-cut | Up to 100 mm |
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| Outer diameter | 6-15 mm | | Inner diameter | 3-6 mm | | Thickness | >=0.15 mm |
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