Crystal Structure and Microwave Dielectric Propertiesof LaLuO3 Ceramics

Jobin Varghese, Tony Joseph, and Mailadil Thomas Sebastianw

Materials and Minerals Division, National Institute for Interdisciplinary Science and Technology (CSIR),Trivandrum 695019, India

Nik Reeves-McLaren

Department of Engineering Materials, The University of Sheffield, S1, 3JD, Sheffield, U.K.

Antonio Feteira

School of Chemistry, The University of Birmingham, Edgbaston B15 2TT, Birmingham, U.K.

Dense LaLuO3 ceramics were prepared by the conventionalsolid-state reaction route. Their room-temperature (RT) crystalstructure and microwave (MW) dielectric properties were inves-tigated. Rietveld refinements performed on X-ray diffractiondata show the RT crystal symmetry to be best described by thecentrosymmetric orthorhombic Pnma space group [a5 6.01888(9) A, b5 8.37489 (12) A, and c5 5.81841 (8) A, Z5 4]. Thisspace group assignment was further corroborated by Ramanspectroscopy analysis. At RT and MW frequencies, LaLuO3

ceramics sintered at 15251C/4 h exhibit a relative permittivity,er 5 22.4, a quality factor, Qu� f5 14 400 GHz (at 5.14 GHz),and temperature coefficient of resonant frequency, sf5�7.5ppm/1C. er corrected for porosity was calculated as 23.9.

I. Introduction

RECENTLY, LaLuO3 was proposed as a potential candidate toreplace SiO2 as a high-k gate dielectric, because of its higher

er, low leakage current, and suitability to be deposited by severalthin-film fabrication techniques such as pulsed laser deposition,molecular beam deposition, and atomic layer deposition. Im-plementation of high-k gate dielectrics has been envisaged as oneof the strategies to further miniaturize microelectronic compo-nents.1,2 These components are also expected to be operating atever increasing frequencies; therefore, they should exhibit goodand stable dielectric characteristics in the GHz frequency range,and in particular low dielectric losses. Similarly to microwave(MW) resonators, a small temperature dependence of the er, te isalso an important requirement for high-k gate dielectrics. It iswell known that in the MW regime, te follows the simple rela-tionship to a first approximation: tf 5�1/2(te1a) [ppm/1C],where tf and a are the temperature coefficient of resonant fre-quency and the coefficient of thermal expansion, respectively.For most perovskites, a is in the order of 10–15 ppm/1C,whereas tf may vary substantially due to the occurrence ofstructural phase transitions, such those involving rotation ofthe octahedra. In fact, tf appears to be strongly correlated withthe departure from the ideal cubic perovskite, as a result of ro-tation of octahedra. This correlation follows a relationship be-

tween tf and the tolerance factor, t, as described by Reaney andIddles3

Although, some interlanthanides have been suggested as po-tential candidates for high-k gate dielectrics, surprisingly theirhigh-frequency dielectric properties remain unexplored. In real-ity, to the best of our knowledge, only the MW dielectric prop-erties of LaYbO3 have been reported. According to Feteiraet al.,4 the MW dielectric properties of LaYbO3 are er5 26,Qu� f5 20 613 GHz, and tf 5�22 ppm/1C. In comparison withother simple perovskites, such as LaGaO3 and LaAlO3, Qu� ffor LaYbO3 is remarkably inferior, which was believed to arisefrom a combination of facts, such as a heavily distorted per-ovskite structure and the presence of a magnetic ion (Yb31),with the latter leading to increasingly higher dielectric losses oncooling.4

In this communication, we report the room temperature (RT)structure and MW dielectric properties (er, Qu� f, and tf) of theLaLuO3. Similarly to LaYbO3, Qu� f for LaLuO3 is less thanhalf of the value reported for LaGaO3, which is also an ortho-rhombic perovskite, whose internal symmetry is described by thePnma space group, but which is less distorted from an ideal cu-bic perovskite. In the case of LaLuO3, the possible deleteriousinfluence of a magnetic ion such in LaYbO3 can be discarded.Hence, it appears that the large departure from an ideal cubicperovskite structure may play a significant role in the intrinsicdielectric losses, providing that extrinsic factors leading to elec-trical conductivity have been minimized.

II. Experimental Procedure

LaLuO3 ceramics were prepared by the solid-state reactionroute. Equimolar amounts of La2O3 (99.5%, Indian RareEarths Limited, Kerala, India) and Lu2O3 (99.9% Sigma Ald-rich, St. Louis, MO) were ball milled for 24 h in a polyethylenebottle using Y-stabilized ZrO2 balls and deionized water. La2O3

powder is hygroscopic, and therefore before weighing it washeated in air at 8001C. The mixed powder was dried and calci-ned successively at 13001C for 4 h and at 14001C for 4 h. Thecalcined powder was ground and mixed with the 4 wt% poly-vinyl alcohol (BDH Lab Suppliers, Poole, U.K.) and shapedinto disks of 14 mm diameter and B7 mm height under an ap-plied pressure of 150 MPa using a WC die. The green compactswere heated in air at a rate of 61C/min up to 6001C and held atthis temperature for 30 min for binder removal. The pellets weresintered in air at temperatures in the range 14751–15501C for4 h. The cooling rate after sintering was 2.51C/min until it

R. Moreira—contributing editor

wAuthor to whom correspondence should be addressed. e-mail: mailadils@yahoo.comManuscript No. 27929. Received April 28, 2010; approved May 12, 2010.

Journal

J. Am. Ceram. Soc., 93 [10] 2960–2963 (2010)

DOI: 10.1111/j.1551-2916.2010.03930.x

r 2010 The American Ceramic Society

2960

reached 8001C and was then allowed furnace cooling. The sin-tered samples were finely powdered by continuous grinding us-ing an agate mortar and pestle. Phase purity and crystalstructure of LaLuO3 were characterized by the X-ray diffrac-tion (XRD) using CuKa radiation (Philips X’pert PRO MPDXRD; Philips, Eindhoven, the Netherlands) operated at 40 kVand 30 mA (a step size of 0.0171 2y). Rietveld refinement wascarried out using the EXPGUI interface for the GSAS softwarepackage.5 The ceramic microstructure was examined using ascanning electron microscope (JEOL-SEM 5600LV, Tokyo,Japan). The stoichiometry of LaLuO3 was evaluated byenergy-dispersive spectroscopy (EDS) analysis carried using atransmission electron microscope (FEI Tecnai G2 30S-TWIN,FEI Company, Hillsboro, OR) operated at 300 kV. Ramanspectroscopy analysis were carried out using a laser Raman mi-croscope (In Via, Renishaw Plc, Gloucestershire, U.K.) with a532 nm line of an Ar1 ion laser as the excitation line.

The bulk densities of the polished samples were measuredusing the Archimedes’s method. Sintered and polished sampleswere used for MW dielectric property measurements using anAgilent Network Analyzer (Agilent Technologies Inc., model8753 ET, Palo Alto, CA) with an invar resonant cavity. Theinterior of the cavity is coated with silver and a low-loss quartzspacer is used for supporting the ceramic. Both er and unloadedquality factor (Qu) were measured by the resonance cavitymethod6 using the TE01d mode of resonance coupled throughE-field probes as described by Courtney.7 The measurementswere made in the frequency range of 4–6 GHz. tf was measuredby noting the variation of resonant frequency of the TE01d modein the reflection configuration over the temperature range of251–751C.

III. Results and Discussion

The RT XRD pattern for LaLuO3 fired at 15501C for 4 h isshown in Fig. 1. All reflections can be indexed according to asingle-phase orthorhombic perovskite suggesting the completereaction between La2O3 and Lu2O3. EDS results also confirmed

a La/Lu ratio of 1. Rietveld refinement of the XRD data wascarried out on two space groups, the centrosymmetric Pnma (no.62) and the non-centrosymmetric Pna21 (no. 33). The extinc-tions expected for both space groups are identical; therefore,they are only distinguishable by the intensity of the reflections. Asimilar situation was reported by Feteira et al.4 for LaYbO3,who found the best fit for their X-ray and neutron diffractiondata to be obtained in the Pnma space group.

A summary of the atomic positions and Uiso values of therefinement of the X-ray data for LaLuO3 using the Pnma spacegroup is given in Table I. The profile difference between the ex-perimental and calculated patterns is shown in the bottom partof Fig. 1. Lattice parameters were calculated as a5 6.01888 (9)A, b5 8.37489 (12) A, and c5 5.81841 (8) A and the reliabilityfactors as Rwp5 18.9%, Rp5 13.9%, and w25 2.04. Similaranalysis was performed in the Pna21 space group and the re-sults are also given in Table I. In this case, the lattice parameterswere calculated as a5 6.01887 (8) A, b5 5.81843 (8) A, andc5 8.37493 (12) A with Rwp5 19.3%, Rp5 13.7%, andw25 2.13. In both refinements, the atomic sites were assumedto be fully occupied and the Uiso values were allowed to refinefreely. A comparison of the reliability factors shows a slightlybetter fit for the Pnma space group. The difference is marginal;however, considering that Lu31 has an ionic radius of 0.861 A,which is very similar to that of Yb31 (0.868 A),8 it is reasonableto expect LaLuO3 to be isostructural with LaYbO3. Moreover,the refinement in the Pna21 contained several unfeasibly shortLu–O bond lengths and negative thermal parameters for theO(2) and O(3) atoms.

Lu–O bond lengths for LaLuO3 refined in the Pnma rangefrom 2.179 to 2.241 A and O–Lu–O angles range from 86.91 to93.11, Table II, showing that LaLuO3 consists of slightly de-formed LuO6 octahedra tilted around the lutetium ions. Therotation of the LuO6 octahedra reduces the coordination num-ber of the La from 12 to 8, and the structure can be described interms of Glazer’s notation as a�b1a�. This distortion awayfrom a perfect cubic perovskite can also be described in terms of

Fig. 1. Experimental, calculated, and difference profiles of X-raydiffraction data for LaLuO3 sintered at 15251C/4 h.

Table I. Summary of the Atom Positions and Uiso Values in Space Group Pnma and Pna21

Space group Atom x y z FRAC Uiso

Pnma La 0.0542 (4) 0.2500 �0.0135 (7) 1 0.0126 (9)Lu 0.5000 0.0000 0.0000 1 0.0140 (8)O(1) 0.4460 (41) 0.2500 0.1000 (42) 1 0.0052 (91)O(2) 0.3092 (31) 0.0541 (25) 0.6784 (33) 1 0.0038 (69)

Pna21 La 0.0532 (5) �0.4872 (9) 0.4827 (23) 1 0.0140 (9)Lu �0.0085 (13) 0.0019 (20) 0.7402 (22) 1 0.0145 (8)O(1) 0.3368 (79) �0.1338 (85) 0.7092 (45) 1 0.0389 (178)O(2) 0.2981 (54) �0.1993 (58) 0.3043 (35) 1 �0.0346 (101)O(3) 0.0617 (43) �0.1026 (47) 1.0151 (48) 1 �0.0088 (91)

Table II. Summary of Lu–O Bond Lengths and O–Lu–OBond Angles in Space Group Pnma

Bond type Bond length (A)Lu–O(1)� 2 2.197 (7)Lu–O(2)� 2 2.179 (19)Lu–O(2)� 2 2.241 (19)

Bond type Bond angle (1)O1–Lu–O1 179.972 (0)O1–Lu–O2� 2 86.9 (8)O1–Lu–O2� 2 87.3 (9)O1–Lu–O2� 2 92.7 (9)O1–Lu–O2� 2 93.1 (8)O2–Lu–O2� 2 89.87 (34)O2–Lu–O2� 2 90.13O2–Lu–O2� 2 180.0

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tilting of the LuO6 octahedra about the [101]p and [010]p axes.Hence, the two identical antiphase tilts around [100]p and [001]pcan be represented by a single tilt y about one of the [101]p diadaxes of the octahedra, i.e. it can be viewed as an independent tiltabout the a axis of the orthorhombic Pnma cell. This tilt angle iscalculated as y5 arcos (c/a)5 14.831. The tilt f about [010]pcorresponds to a single in-phase rotation of the octahedra aboutthe b axis of the orthorhombic Pnma cell. This tilt angle can becalculated as f5 arcos (

ffiffiffi

2p

c=b)5 10.731. Both y and f rota-tions can be combined into a single tilt F about the [111]p axisand it can be calculated as F5 arcos(

ffiffiffi

2p

c2=ab)5 18.231. Thecalculated values are similar to those reported for LaYbO3

(y5 14.71, f5 11.11, andF5 18.41), but much larger than thosefor LaGaO3 (y5 6.11, f5 2.11, and F5 6.41).

Figure 2 shows the typical ceramic microstructure of LaLuO3

ceramics sintered at 15251C/4 h. This intergranular fractureshows a dense microstructure with grain sizes ranging from 5to 10 mm. The sample sintered at 15501C has partially melted.The RT Raman spectrum for LaLuO3, Fig. 3, exhibits the samespectral features reported for LaYbO3, for which the internalcrystal symmetry at RT is described by the Pnma space group.9

Factor group analysis predicts a total of 57 and 24 Raman-ac-tive modes for the Pna21 and Pnma space groups, respectively.The expected 24 Raman-active modes for a crystal with sym-metry described by the Pnma space group are 7Ag, 5B1g, 7B2g,and 5B3g; however, often they are not all observed as pointedout by several workers. The temperature dependence of theRaman spectra for LaLuO3 will be reported elsewhere.

The variation of relative density and er with the sinteringtemperature is illustrated in Fig. 4(a). The relative density ofLaLuO3 increased from 87.5% toB96% for compacts fired for4 h at 14751 and 15251C, respectively. No significant densificat-ion was observed at temperatures greater than 15251C. A similar

trend was observed for er dependence with the sintering tem-perature. er of LaLuO3 ceramic sintered at 14751C is 17.2 andincreased to a maximum value of 22.4 for ceramics fired at15251C. This er enhancement results from the decrease of theporosity. The value of er corrected for porosity was calculated as23.9 using the equation derived by Penn et al.10

The dependence of Qu� f and tf for LaLuO3 ceramics withthe sintering temperature is illustrated in Fig. 4(b). Qu� f valuesincreased with the sintering temperature reaching a maximum ofB14400 GHz for ceramics fired at 15251C. Qu� f decreases forceramics fired above 15251C because of partial melting and ap-pearance of cracks. tf decreased with sintering temperature andbecame almost a constant of B�7.5 ppm/1C for ceramics firedat 15251C. In summary, LaLuO3 ceramics sintered at 15251C for4 h exhibit er 5 22.4, Qu� f5 14400 GHz, and tf 5�7.5 ppm/1C. Both er andQu� f values for LaLuO3 are slightly lower thanthose reported for LaYbO3, whereas tf is better in the case ofLaLuO3. Among other possible reasons, the difference onQu� fmay be due to the slightly higher relative density attainedfor LaYbO3 ceramics. Finally, it is worth mentioning thatboth LaLuO3 and LaYbO3 exhibit much lower Qu� f valuesthan LaGaO3, which is a less distorted perovskite as indicatedby the tilt angles.

IV. Conclusions

LaLuO3 ceramics were prepared by the conventional ceramicroute. Rietveld refinement analysis of XRD data for LaLuO3

revealed the crystal structure symmetry to be described by thecentrosymmetric Pnma space group (no. 62). The lattice param-eters were calculated as a5 6.01888 (9) A, b5 8.37489 (12) A,and c5 5.81841 (8) A. LaLuO3 sintered at 15251C for 4 h exhibiter5 22.4, Qu� f5 14 400 GHz, and tf 5�7.5 ppm/1C. In com-parison with other less distorted orthorhombic perovskites suchas LaGaO3, Qu� f values for LaLuO3 are substantially lower,

Fig. 2. Scanning electron microscopy image of the fractured surface ofLaLuO3 ceramics sintered at 15251C/4 h.

Fig. 3. RT Raman spectrum for LaLuO3.

Fig. 4. Variation of (a) relative density and relative permittivity; (b)Qu� f and tf as a function of sintering temperature.

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suggesting that the degree of octahedra rotation may play animportant role in the intrinsic losses.

Acknowledgments

Mr. Jobin Varghese is thankful to Mr. V. Velmurugan, Division Leader,Nanoelectronics Division, VIT, Vellore, Tamil Nadu, India. Mr. Tony Joseph isgrateful to the Council of Scientific and Industrial research, India for the researchfellowship. The Raman microscope used in this research was obtained throughBirmingham Science City: Creating and Characterising Next Generation Ad-vanced Materials (West Midlands Centre for Advanced Materials Project 1),with support from Advantage West Midlands (AWM) and partly funded by theEuropean Regional Development Fund (ERDF).

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