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Unusual negative longitudinal piezoelectric effect (NLPE) and electric auxetic effect (EAE) have essential implications for designs of piezoelectric sensors and actuators. The emerging ferroelectric HfO2 is recently discovered to have both effects, while the underlying physical mechanisms remain elusive. To understand and regulate these intriguing effects, it is crucial to investigate the piezoelectricity in ferroelectric HfO2 and related fluorite-structure ferroelectrics. Here, we corroborate using first-principles calculations that all twelve fluorite-structure ferroelectrics covered in this study possess the NLPE. A chemical tendency of piezoelectricity is demonstrated, i.e., the larger the "iconicity," the stronger the NLPE. The structural origin is attributed to the predominant influence of the triple-coordinated anion displacement, namely, the more "ionic" fluorite-structure ferroelectrics exhibit larger anion displacement under a pressure or strain, which gives rise to a more negative internal-strain contribution dominating over the positive clamped-ion contribution and hence a stronger NLPE. Moreover, we confirm several electric auxetic materials in fluorite-structure ferroelectrics with finite electric field calculations. We find that the piezoelectricity of electric auxetic materials is suppressed by the external electric field along the polar direction, since it weakens the bonding heterogeneity. The unraveled fundamental understanding of the NLPE and EAE in this study may profoundly benefit the design and application of fluorite-structure ferroelectrics. [ABSTRACT FROM AUTHOR]

This work deals with enhancing piezoelectric energy harvesting by incorporating different auxetic structures and varying geometrical parameters. A bimorph consisting of polyvinylidene fluoride as a piezoelectric layer and brass auxetic substrate (re-entrant, s-shape, and elliptical holes) are considered for analysis. A finite element method-based simulation is performed to find the best auxetic systems that provide higher power output. The environmental vibration is taken for energy harvesting; thus, the first resonance frequency is below 100 Hz. The maximum power of ∼0.52, ∼0.67, and ∼0.79 mW is estimated for piezoelectric energy harvesters (PEH's) re-entrant, s-shape, and elliptical holes. The auxetic re-entrant, s-shape, and elliptical substrate-based PEHs show 200%, 300%, and 333% more power than the solid substrate (conventional design). However, the elliptical-based auxetic structure obtains a maximum power density of 0.004 66 mW/mm3. The stresses in all structures are within the permissible limit; hence, any design can be used for practical applications. All three auxetics have comparable geometrical dimensions and the same material is used; thus, auxetic behavior is independent of the material employed and depends on the structure's shape. The estimated power is higher than that reported in the literature. [ABSTRACT FROM AUTHOR]

Using first-principles calculations, we propose a new two-dimensional Ge2S (space group P21212) with unique mechanical and electronic properties. Monolayer Ge2S has excellent thermal, mechanical, and dynamic stabilities, exhibiting a semiconducting behavior with an indirect bandgap and anisotropic carrier mobility. The uniaxial strain along the zigzag direction can induce an indirect-to-direct bandgap transition. Remarkably, Ge2S possesses large in-plane negative Poisson's ratios, comparable with that of well-known penta-graphene. Moreover, we identify Ge2S as a high-performance anode material for metal-ion batteries. It shows metallic features after adsorbing Na, K, and Mg, providing good electrical conductivity during the charge/discharge process. The diffusion of metal ions on Ge2S is anisotropic with modest energy barriers in the armchair direction of 0.12, 0.39, and 0.76 eV for Na, K, and Mg, respectively. Ge2S can adsorb metal atoms up to a stoichiometric ratio of 1:1, which yields storage capacities of 151.17, 151.17, and 302.35 mA h g−1 for Na, K, and Mg, respectively. The volume of Ge2S shrinks slightly upon the adsorption of metal ions even at high concentrations, ensuring a good cyclic stability. Besides, the average open circuit voltage (0.30–0.70 V) falls within the acceptable range (0.1–1.0 V) of the anode materials. These results make Ge2S a promising anode material for the design of future metal-ion batteries. [ABSTRACT FROM AUTHOR]

A theoretical model has been developed to express the longitudinal and transverse piezoelectric effects in ferro/piezoelectrets. This model is an extension of the classic model based on layer-dielectric structures that focuses on the longitudinal piezoelectric effect. The improved model reveals that the relation between the longitudinal piezoelectric d33 coefficient and the transverse piezoelectric d31 coefficient is associated with Poisson's ratio μ13 (or μ31) and Young's moduli Y1 and Y3. The polarity of the two coefficients is opposite for materials with a positive Poisson's ratio as normally observed in conventional piezoelectric materials, and the same for materials with a negative Poisson's ratio, i.e., auxetic materials. The experimental results available in the literature and obtained from ferroelectret metamaterials with a honeycomb cross-sectional structure and two-dimensional (2D) symmetric structure, called 2D ferroelectret metamaterials, designed and prepared in this study reasonably agree with theoretical predictions. This study not only extends the scope of the metamaterials by inducing the concept of ferroelectret metamaterials but also provides a strategy for designing ferroelectrets with designable properties. [ABSTRACT FROM AUTHOR]

The purpose of this research is to analyze the mechanical behavior of auxetic re-entrant-based metamaterials with properties similar to Inconel 625 using homogenization techniques. Through a thorough analysis, this study investigates the displacement patterns exhibited in various materials throughout a range of thicknesses. The examination also includes analyzing how the Young's modulus changes with varying strut thickness after homogenization. This detailed investigation provides information on the stiffness and deformation response of the material. This research advances our knowledge of the complex mechanical properties of re-entrant-based auxetic metamaterials that resemble Inconel 625 by interpreting these displacement and Young's modulus patterns. [ABSTRACT FROM AUTHOR]

Recent successes in the discovery of novel two-dimensional (2-D) phosphorene allotropes have motivated more in-depth investigations into tuning their properties through precise geometric control. This is also driven by the fact that these materials, particularly blue phosphorene, are highly prone to wrinkling. In this work, we systematically study the mechanical and electronic behaviors of a series of rippled blue phosphorene PN (N = 8, 18, 32, 50, 72, 98) using density functional theory combined with molecular dynamic simulations. A novel approach to tailor the electronic energy band structure of blue phosphorene is proposed by wrinkle engineering, transforming the native indirect bandgap into a direct bandgap, and enabling bandgap tuning by modifying the undulation magnitude ratio. Furthermore, the mechanical behaviors of rippled blue phosphorene differ significantly along the 4-8-4 and 4-4-4 directions. Notably, negative Poisson's ratio is observed under tension along the 4-4-4 direction. This work demonstrates new techniques for geometrically regulating blue phosphorene and potentially other 2-D materials. The findings also yield valuable insights for the design of novel 2-D auxetic semiconductors with tunable electronic properties. [ABSTRACT FROM AUTHOR]

Superhydrophobicity plays a pivotal role in numerous applications. Recently, we have demonstrated the potential of auxetic metamaterials in creating superhydrophobic materials with unique wetting properties. However, the superhydrophobic properties are lost when the liquid penetrates into the surface structure. Understanding the conditions for droplet penetration is crucial for advancing wetting control. Here, we experimentally identify the transition from droplet suspension to full-penetration on an auxetic bowtie/honeycomb lattice membrane. We develop a comprehensive physical model surface representing different states of strain, ranging from auxetic to conventional lattice membranes, and consider the wetting as the liquid surface tension is varied using water/ethanol mixtures. By examining the interplay between contact angle and lattice structure, we gain valuable insights into the conditions for droplet suspension and full-penetration. Additionally, we develop a simple touch test to discern whether a droplet has effectively fully penetrated the structure, providing a practical and efficient means of distinguishing the different wetting states (suspended or partially penetrating vs fully penetrating). [ABSTRACT FROM AUTHOR]

The use of metamaterial structures with auxeticity can result in exceptional mechanical properties, such as high energy absorption and fracture resistance. However, traditional design approaches rely heavily on researchers' subjective experiences, while existing inverse design methods limit design possibilities by ignoring generative diversity. In this study, we report a deep-learning-based inverse design approach for 3D auxetic unit cells that overcomes these limitations by providing diverse and accurately conditioned design options. We construct a dataset of symmetric 3D auxetic unit cells and apply an elastic modulus optimization network to generate diversified spatial topological structures with negative Poisson's ratios and optimized stiffness. The resulting 3D unit cells exhibit improved mechanical properties, as confirmed by finite element simulations and experiments. Our approach offers better coverage of the design space and generates optimized 3D unit cells with rich and diverse properties. [ABSTRACT FROM AUTHOR]

In this Letter, the structural, mechanical, electronic, and transport properties of two-dimensional (2D) ternary Janus Si 2 X Y (X / Y = S, Se, Te) monolayers are studied based on the calculations using first-principles density functional theory. All three structures are found as direct semiconductors with moderated bandgap energies and good stabilities for experimental synthesis. The transport properties are also examined by calculating the carrier mobilities. We find that the carrier mobilities of all three monolayers are anisotropic not only between the electrons and holes but also between the two transport directions. The Si2SSe monolayer exhibited the highest electron mobility of 897.66 cm2 V−1 s−1 in the x axis. Amazingly, our calculations reveal that the Si 2 X Y monolayers are auxetic materials with negative Poisson's ratio along both x and y axes. Particularly, the Si2SSe has the largest negative Poisson's ratio value of –0.131 in the x direction. These obtained results open more prospects for advanced applications of these materials in electronics, optoelectronics, and nanomechanics. [ABSTRACT FROM AUTHOR]

This document is a summary of a special issue on superhydrophobic surfaces published in the journal Applied Physics Letters. Superhydrophobic surfaces are engineered to repel water efficiently, and they have various applications such as anti-icing, self-cleaning, and liquid manipulation. The special issue includes research on topics such as the capture of air by superhydrophobic materials, droplet evaporation on these surfaces, the development of durable superhydrophobic surfaces, and the expansion of their functionality through macrostructures and surface wettability patterns. The issue also covers the dynamic interactions between liquid droplets and solid surfaces, the stability and durability of superhydrophobicity, and the integration of superhydrophobic properties with other effects to create multifunctional surfaces. Overall, the special issue provides a comprehensive overview of the current research and potential applications of superhydrophobic surfaces. [Extracted from the article]

Hardness and fracture toughness are some of the most important mechanical properties. Here, we propose a simple model that uses only the elastic properties to calculate the hardness and fracture toughness. Its accuracy is checked by comparison with other available models and experimental data for metals, covalent and ionic crystals, and bulk metallic glasses. We found the model to perform well on all datasets for both hardness and fracture toughness, while for auxetic materials (i.e., those having a negative Poisson's ratio), it turned out to be the only model that gives reasonable hardness. Predictions are made for several materials for which no experimental data exist. [ABSTRACT FROM AUTHOR]

As interesting analog of black phosphorus, group-IV monochalcogenides MX (M = Ge, Sn; X = S, Se) are greatly expected to have potential applications in photodetectors, photovoltaics, thermoelectric, and piezoelectric devices. In this paper, we conduct a thorough investigation on the mechanical properties of four MX monochalcogenides, i.e., single- and multi-layered GeS, GeSe, SnS, and SnSe, including their ideal tensile strength, evolution of monolayer bulking height, and phase transition under uniaxial strain within the framework of density functional theory. The ultimate tensile strengths of the four monochalcogenides are found rather low, while the failure strains are remarkably high, showing ultrahigh mechanical flexibility. Meanwhile, the ideal strength demonstrates strong anisotropy, which may induce the exfoliated MX thin flakes showing an obvious in-plane cleavage tendency of much easier to crack along the zigzag orientation. In addition, it is found that all of them are potential auxetic materials with large negative Poisson's ratios, just like single-layer black phosphorus. Furthermore, we find all the monolayer monochalcogenides will demonstrate geometrical phase transition when uniaxial compressive strains are applied. It is expected that these newly found unique features of monolayer group-IV monochalcogenides and their derivatives make them attractive candidates for their incorporation in future mechanical, nanoelectronics, and thermal applications. [ABSTRACT FROM AUTHOR]

The discovery of auxetic behavior (negative Poisson's ratio) within elements and alloys had focused attention on their elastic anisotropy in an effort to understand the range of crystal orientations that manifest this property. A comparison of elastic constant data to atomistic models based on pair-wise, central force models provides key insights into deformation behavior of cubic crystals over a wide range of anisotropy, including, for the first time, those with Zener anisotropy ratios less than 1. A simple criterion is derived which dictates all cases in which a crystal whose atomic ordering obeys cubic symmetry will display auxetic deformation, where the extrema in Poisson's ratio involves (110) orientations. In the field of stress determination through x-ray diffraction, these findings also shed light on strain anisotropy in polycrystalline materials, where the elastic incompatibility between adjacent grains alters their overall deformation. By applying these same atomistic models, we can predict the Voigt/Reuss weighting fractions associated with Kröner limit x-ray elastic constants for cubic materials, a necessary component in quantifying stress using diffraction data. We also establish that greater elastic anisotropy in a constituent crystal leads to a more rigid mechanical response in the corresponding polycrystalline aggregate, with implications for auxetic crystal ensembles. [ABSTRACT FROM AUTHOR]

Through a combination of analytical, numerical, and experimental methods, we study a three-dimensional metamaterial with the ability to attenuate both airborne sound and mechanical vibrations, simultaneously, and in all directions. In addition, due to the auxetic nature of the design (i.e., having a negative Poisson's ratio), the metamaterial can shrink (or expand) in a relatively uniform manner, without buckling. We utilize an external load to cause a systematic shape change in the metamaterial and tune the attenuation frequency bands. The presented design principles can be utilized in many applications related to acoustic and elastic wave manipulation as well as acoustic devices. [ABSTRACT FROM AUTHOR]

The auxetic effect and topological phase transition are interesting mechanical and electronic properties of some materials, respectively. Although each has been extensively studied separately, no material has been identified to possess both properties simultaneously. Here, we report that a two-dimensional phosphorous nitride monolayer simultaneously possesses auxetic behavior and undergoes a topological phase transition under tensile strain. The monolayer has a normal-auxeticity mechanical phase transition when a tensile strain above 0.055 is applied along the P–P zigzag direction. The negative Poisson ratio can even approach as abnormally high as −0.60. Furthermore, the material is an intrinsic Dirac material, but a phase transition from the semi-Dirac material to Dirac material is observed at nearly the same critical tensile strain as that in auxetic phase transition. An electronic orbital analysis reveals that the simultaneity of the normal-auxeticity phase transition and topological phase transition originates from the variation of orbital hybridization around the Fermi level. [ABSTRACT FROM AUTHOR]

A mechanically reconfigurable reflector (MRR) can adapt its geometry to facilitate a space antenna's ability to scan along different angles. The reflector geometries corresponding to various scanning angles are calculated by coordinate change. The proper mechanical properties of the MRR are important for reconfiguration accuracy and actuation of the reflector system. The ratio of bending stiffness to Young's modulus (D/E) is an essential parameter for the smooth reshaping of the MRR. In previous MMR research, materials with a small Young's modulus were used, and the D/E ratio is increased by increasing shell thickness. In order to increase the D/E ratio while maintaining a thinner shell thickness, this paper proposes an auxetic lattice material for a reconfigurable reflector shell. The auxetic lattice material allows the raw material of the reflector shell to have a high Young's modulus resulting in a smaller thickness. By optimizing the stacking sequence of carbon fiber composite, the bending stiffness of raw material is improved. Compounding the stacking sequence with elliptic voids array structure in reflector shell, the in-plane effective Young's modulus is significantly decreased while maintaining a small decrease in bending stiffness. For a given target shape, the geometric parameters of the elliptic void are determined by the comparison of the reshaping accuracy calculated through simulation. [ABSTRACT FROM AUTHOR]

Structure evolution during deformation of isotropic auxetic foams is investigated by simple compression experiments. It is shown that the main feature observed in the experimental data can be accurately described by an isotropic hyperelastic model with an Ogden-type strain energy function. The model can be easily implemented in a finite element code and, hence, can be used to simulate foams undergoing complex structural deformations. [ABSTRACT FROM AUTHOR]

Two dimensional (2D) auxetic materials with large negative Poisson's ratio (NPR) are highly desirable for applications in medicine, tougher composites, defense, and so on. In this paper, by using first-principles calculations, we identify that the CaAs3 monolayer is an excellent auxetic material with large NPR value up to −0.36 along the hinges direction. The relatively small Young's modulus ranging from 23.61 to 40.50 N/m and the moderate critical strain ranging from 0.06 to 0.16 guarantee its extraordinary flexibility and moderate ductility. Owing to its unique crystal structure, the anisotropy of basic mechanical parameters, electrical conductance, and optical absorption are concentrated on the CaAs3 monolayer. Moreover, the electric field-induced tunability of optical absorption also emerged in the CaAs3 monolayer. All of these extraordinary properties allow 2D CaAs3 to have great potential applications in designing electromechanical devices and photoelectric devices as well. [ABSTRACT FROM AUTHOR]

Materials with a negative Poisson's ratio, also known as auxetics, have attracted a lot attention as they have shown innovation potential in applications for energy damping, modern fasteners, biomedical implants, piezoelectric sensors, and soft actuators. In this study, we introduce different patterns of graded distribution of unit cells with positive and negative Poisson's ratio in tubular configurations. Seven types of patterns are programmed into the fabric of the metamaterial to create desired shape changes upon applying far-field external loads. Two of the patterns demonstrate near-zero Poisson's ratio even at axial strains as high as 44%. Other pattern distributions convert the initially cylindrical shape of the tubes to vase, barrel, hourglass, nonsymmetrical vase, and nonsymmetrical hourglass geometries. The experimental Poisson's ratio values for the linear negative-to-positive-to-negative gradient (resulting in hourglass shape) and linear positive-to-negative-to-positive gradient (resulting in barrel shape) cases are +0.53 and −0.47, respectively. The measured Poisson's ratio values at tube level are in good accordance with the analytical values of +0.5 and −0.5. Benefits of the proposed designs in applications such as action-at-a-distance actuators and wrinkle-free jointless hinges in both 3D and 2D configurations are demonstrated. [ABSTRACT FROM AUTHOR]

In the frame of thermodynamic approach, it is shown that all the admissible values of the d33, d32, and d31 piezoelectric moduli of any orthorhombic (mm2) single crystal in the 3-dimensional (d33, d32, d31) space lie within a restricted area having the form of ellipsoid. The main parameters of this ellipsoid (the aspect ratios and the angles characterizing its orientation) depend only on the elastic constants of the crystal. Using the developed approach, the areas of the allowable values of piezoelectric moduli have been determined for six crystals: single-domain KNbO3 and 0.27Pb(In0.5Nb0.5)O3-0.40Pb(Mg1/3Nb2/3)O3-0.33PbTiO3 crystals possessing true orthorhombic symmetry and four polydomain macroscopically pseudoorthorhombic crystals-tetragonal BaTiO3 and PbTiO3 crystals with laminate 90° domain structure as well as rhombohedral 0.72Pb(Mg1/3Nb2/3)O3-0.28PbTiO3 and 0.93Pb(Zn1/3Nb2/3)O3-0.07PbTiO3 crystals, poled along the [011] direction of the prototype cubic cell. Four crystals from this set possess negative values of the longitudinal Poisson's ratio and are thus partially auxetics. [ABSTRACT FROM AUTHOR]

We demonstrate an effective approach to enhance the output performance of a flexible piezoelectric energy harvester by using flexible electrodes with negative Poisson's ratio (NPR). The relationship between open-circuit voltage and Poisson's ratio of electrodes is established theoretically by deriving the analytical expression. It reveals a continuous increasing trend in open-circuit voltage with the decrease in Poisson's ratio of the electrodes. Further, graphene-assembled macro-film (GAMF), an NPR material with excellent flexibility and high conductivity, is used as the electrodes to fabricate flexible piezoelectric energy harvesters. Compared with the energy harvesters using silver electrode, the harvesters made by GAMF electrodes, with an NPR of −0.39, achieve nearly 1.7-times enhancement in open-circuit voltage and 1.6-times in short-circuit current for output performance. The experimental results are highly consistent with the simulation results, indicating that the GAMF has great prospects in developing flexible piezoelectric energy harvesters with enhanced electrical output performance. [ABSTRACT FROM AUTHOR]

We numerically and experimentally report 3D tubular lattice metamaterials with sinusoidal ligaments, which enable the deformation pattern to switch from bending to stretching. A non-linear J-shape stress-strain response and auxetic behavior over a large deformation range are observed due to the intrinsic deformation mechanism. Numerical analyses further demonstrate that the proposed tubular lattice metamaterials exhibit a high degree of tunability for Poisson's ratio and mechanical responses. In particular, the design flexibility can be harnessed to create heterogeneous tubular structures that can morph into target shapes. The findings presented here provide insights to develop 3D architected metamaterials with various potential applications, such as tissue engineering, biomedical devices, and stretchable electronics. [ABSTRACT FROM AUTHOR]

Auxetic metamaterials have received wide attention due to their idiosyncratic physical properties. In this paper, experiments and simulations were performed to study the mechanical and acoustic properties of a 3D anti-chiral auxetic metamaterial subjected to quasi-static loading. The results shows that Poisson's ratio of the designed metamaterial can be changed from −0.45 to 0.35 with different cell parameters. Acoustic tests show that the designed metamaterial provides an ultra-wide phononic bandgap and exhibits outstanding wave attenuation properties. By applying rational tensile/compression force, the bandgap is widened/narrowed, which indicates the tunability of the bandgap through deformation. Moreover, the bandgap tuning is through all high symmetry points in the Brillouin zone due to the iso-auxeticity of the metamaterial. This study provides a possible way for controlling the bandgap with the deformable metamaterial. [ABSTRACT FROM AUTHOR]

Elastic bulk materials support longitudinal and transverse waves such that the former travels faster in most cases. Anomalous polarization is the case when a transverse wave travels faster, allowing us to engineer the wave propagation via wave steering, scattering control, and mode conversion, which has critical applications in vibration mitigation and ultrasonic imaging. However, realizable materials that exhibit anomalous polarization are rarely found in nature, and architected materials that exhibit this property have only been demonstrated in 2D. In this article, we present 3D auxetic periodic lattice materials that support anomalous wave polarization. Through finite element analysis, we show that these lattices can switch between normal and anomalous behavior via simple geometry changes. We confirm the elasticity condition and qualitatively discuss the guidelines to design lattice materials that support anomalous polarization along a specific wave propagation direction. We show the ability to control the anomalous wave propagation direction by modifying the lattice geometry. Further, we numerically demonstrate mode conservation, deceleration, and acceleration of an incident wave using a material that exhibits anomalous wave polarization. These demonstrations show the potential application of such materials in nondestructive evaluation and medical imaging. [ABSTRACT FROM AUTHOR]

The emergence of advanced 3D printing techniques and the recent interest in architected materials have sparked a surge of interest in mechanical metamaterials whose unusual properties are defined by their highly ordered microarchitectures. Mechanical metamaterials with disordered microarchitectures have, however, not received as much attention despite their inherent advantages, such as robustness against the precise arrangement and design parameters of individual unit cells. Here, we computationally studied the elastic properties of two general types of disordered networks, namely, lattice-restricted and unrestricted networks that were made of beamlike elements and possessed mean connectivity values, Z, ranging between 2.5 and 7. We also additively manufactured a number of representative networks using selective laser sintering and showed that their deformations are consistent with our computational predictions. Unrestricted networks exhibited several advantages over the lattice-restricted ones including a broader range of achievable elastic modulus-Poisson's ratio duos as well as a higher probability of exhibiting auxetic and double-auxetic (i.e., auxetic behavior in both orthogonal directions) behaviors. Most interestingly, we could find unrestricted auxetic networks for high connectivity levels of up to 4.5, while no lattice-restricted auxetic networks were found for any connectivity level beyond 3.5. Given the fact that, according to Maxwell's criterion, 3.5 is the highest Z for which both of our lattice-restricted and unrestricted networks are bending-dominated, we concluded that unrestricted networks exhibit auxetic behavior well into their stretch-dominated domain. This is a promising observation that underlines the potential of unrestricted networks for the challenging task of designing stiff auxetic metamaterials in the stretch-dominated domain (i.e., Z = 4–4.5). [ABSTRACT FROM AUTHOR]

Auxetic materials (materials with negative Poisson's ratio) and nanomaterials have independently been, for many years, two of the most active research fields in material science. Recently, these formerly independent fields have begun to intersect in new and interesting ways due to the recent discovery of auxeticity in nanomaterials like graphene, metal nanoplates, black phosphorus, and others. Here, we review the research emerging at the intersection of auxeticity and nanomaterials. We first survey the atomistic mechanisms, both intrinsic and extrinsic, that have been found, primarily through atomistic simulations, to cause auxeticity in nanomaterials. We then outline the available experimental evidence for auxetic nanomaterials. In order to lay the groundwork for future work in this exciting area, we close by discussing several future prospects as well as the current challenges in this field. [ABSTRACT FROM AUTHOR]

We investigate the vibrational properties of superlattices with layers of rubber and polyurethane foam, which can be either conventional or auxetic. Phononic dispersion calculations show a second pass band for transverse modes inside the lowest band gap of the longitudinal modes. In such a band, the superlattices behave as a double-negative elastic metamaterial since the effective dynamic mass density and shear modulus are both negative. The pass band is associated to a Fabry-Perot resonance band which turns out to be very narrow as a consequence of the high contrast between the acoustic impedances of the superlattice components. [ABSTRACT FROM AUTHOR]